U1241B multimeter woes - Page 1
Forum section: electronics / testgear

author: najrao
author_rank: Regular Contributor
timestamp: November 11, 2017, 12:17:58 pm
content: On opening my Agilent U1241B which would not read ohms, I discovered that the rotary switch contact pads were burnt out and the board scorched badly in the area. I failed to take a picture then, but did one after some restoration.Analyzing what could have caused such a disaster in an expensive instrument made by a highly acclaimed maker, I discovered to my horror that the switch, with an interpad clearance of just 0.8mm has to break 1000V rms, and remains exposed to the full voltage at the VOLTS input terminal at all times. The VOLTS selector itself has much better a clearance of 3mm, and uses a double width shorting contact.Much correspondence with Keysight has produced no worthwhile explanation. They keep harping on how the 'incident' is unique in their databank, is 'unfortunate', and continue to assert that their product is free of defect and complies with all requirements of certification. Critical questions are just ignored: e.g., " How do the Ohms/Diode/Capacitance ranges withstand the claimed 1000Vrms with just the 1k resistor and 22ohm PTC as protection elements? The tracks on the measurement side of the switch are spaced 0.25mm apart, and the various extremely narrow plated vias would vaporize with the ensuing current."They have made a peace offering: to replace the damaged instrument!These problems must exist in every other 1241, and may extend to many other models as well. I want to warn users of the serious design defects which the maker has let pass -- but won't admit to.This post is necessarily very sketchy, but would have to be many pages long to convey the full implications. Interested users please contact me for much more detail.

author: bd139
author_rank: Super Contributor
timestamp: November 11, 2017, 12:22:52 pm
content: Well I was about to grab a 1241C. Certainly not going to now. Thanks for your time to detail this.Anyone care to suggest a safer alternative for around the same price point?

author: Terry01
author_rank: Frequent Contributor
timestamp: November 11, 2017, 01:02:28 pm
content: Why didn't you pack it up and send it straight back if it was so badly damaged? Sounds like someone else has used it before you if it arrived in that state.How can they argue if the meter arrives wrecked like that.

author: Fungus
author_rank: Super Contributor
timestamp: November 11, 2017, 01:11:09 pm
content: Quote from: najrao on November 11, 2017, 12:17:58 pmI'm not sure what clearance you're talking about, but it doesn't matter what the clearance of an individual thing is, so long as there's a bigger clearance further down the line.Quote from: najrao on November 11, 2017, 12:17:58 pmWhat "1k resistor"? Have you got a schematic?Those meters are third party certified by CSA. If it says "CAT III 1000V" on the front then it's CAT III 1000V.(although CAT rating doesn't mean it has to survive, it's allowed to fail safely if that manufacturer wants it to)Quote from: najrao on November 11, 2017, 12:17:58 pmWhy the big mystery? Post the details here. Plenty of meters have been roasted in these forums.eg. What conditions was it exposed to?

author: najrao
author_rank: Regular Contributor
timestamp: November 12, 2017, 09:56:47 am
content: Thanks everybody for the responses.First, I attach the photo, now compressed, hope it won't be rejected for size. Note this was taken after my repair.The 'switch pad clearance' is the spacing between the two inner tracks. The main hot input terminal is connected to the outer of the two through the protection network. The moving contact connects it to the inner track, for each of three switch positions: Ohms, Diode, and Capacitance. This switch remains open in all other positions. The inner pad/s of this switch connect through (at least) one very small pth via, visible close to the central guide hole. Since intertrack clearances beyond this point are very small indeed, less than 0.25mm, I conclude that potentials here are at most a few volts.Note that ANOTHER identical protection network is present, connected to the hot input again. This feeds the voltage measurement circuit, switched on the outer tracks and offering much larger an intertrack clearance. As much as 3mm. The shorting contact for this is also different, at twice the pitch. It is clear that the designer is entirely aware of the need to space tracks wider apart, but fails to implement this in the Ohms circuit even though the voltage to be stood off is the same 1000V.The protection network in each case has a 1k, 1 possibly 2W series resistor; it's speciality if any is unknown, but I presume it is a special 'fusible' resistor a la Fluke. Its thermal capacity is still far too high in my opinion to limit the energy let through into the inner measuring circuit, unless this latter can clamp its voltage to under a few volts, limiting the power, while the protection resistor passes up to 250mA before opening up.The two spark gap 'diodes' cannot and should not have a role to play for impressed voltages of up to 1000V rms, more if the permitted crest factor is taken into account. The PTC thermistors would act to curtail the fault current, but they too have large heat capacity and would need time.All these protection components are soldered in place, unlike the two big fuses for the current measurement circuits. Operation of any one or more soldered component would render the instrument unusable, or in any case require factory service. Thus, the unit can not be said to 'withstand 1000V rms', even if the Cat III certificate would appear to say so.I have explained all these points threadbare to Keysight's 'site assistance'. To summarize their answers:1. Never happened before, unique case in our data bank; unfortunate2. The U1241B complies entirely with its declared specification and is certified to CATIII3. Give us the failed unit, we will investigate further4. All manufacturers made design changes in 2011 to comply with revisions to standards; my unit is earlier. Will replace if I so want5. All U1241B's will withstand 1000V rms at the hot input terminal with the switch in any of the positions measuring Volts, Ohms or Capacitance.I have even asked their concurrence for me to test my unit with voltage applied in steps starting at say 300V.Their replies do not even pretend to a answer the points raised. I can forward the correspondence by private mail if this is useful.

author: najrao
author_rank: Regular Contributor
timestamp: November 12, 2017, 10:53:55 am
content: I must have been dropped on my head as a baby by my mother: I just can't attach the pictures, even after compressing them. The server has rejected the attachment again. What do I do?Am trying again, with even more compression.

author: bd139
author_rank: Super Contributor
timestamp: November 12, 2017, 11:52:33 am
content: Upload them to imgur.com and insert the provided bbcode links. Works much better!

author: Fungus
author_rank: Super Contributor
timestamp: November 12, 2017, 12:47:48 pm
content: Quote from: najrao on November 12, 2017, 09:56:47 amIf the meter can measure 1000V then none of the safety components should do anything until way beyond that.Quote from: najrao on November 12, 2017, 09:56:47 amI've got a $3 multimeter that has correctly measured up to 1000V. I have no doubt yours can withstand that much. The CAT ratings are for what happens beyond.Here's the CAT ratings chart. There was a second edition to add CAT IV. Does your meter mention CAT IV on the front?Note that CAT III 1000V is the same as CAT IV 600V. CAT III 1000V allows for an 8000V spike with 2 Ohms impedance, ie. 4000Acouldpass, if the device allows it to.Note that a CAT rating doesn't mean the meter has to survive, it's allowed to die so long as it does it safely (although the big hole in your PCB doesn't look 'safe' to me).In tests, our joeqsmith managed to kill a Keysight U1231A in a similar way:https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/msg852693/#msg852693Your input protection looks similar to the U1231A. Joe managed to get arcing across the rotary switch contacts just like you're seeing but it took 5000V to do it.Quote from: najrao on November 12, 2017, 09:56:47 amKeysight certifies their meters independently at great expense. Their tech support people have no reason to believe a random internet user over their certification process. They don't know what the meter was exposed to, if there was contamination inside, or whatever.You haven't told us what happened to your meter. What were you doing when it failed?

author: najrao
author_rank: Regular Contributor
timestamp: November 12, 2017, 02:29:43 pm
content: I can not be sure what were the conditions just at the time of failure. I was not using it, I and one who was did not report any unusual circs. I opened it when it died on Ohms, to see what.It is just possible someone measured a small elco without discharging it, but it cd not if bn more than 100u@150V.I should add that there is no issue of the instrument being unable to measure 1000V on the two VOLTS ranges: there is a string of 5×2M MELF resistors coming up in series immdtly after the protection circuit, and these are located, spaced and connected up with generous clearances as should.I agree entirely that the protection components should have NO role to play whatsoever when the unit is operated within its ratings.The two main defects are:1. The switch pads for the Ohms range are too close to each other (0.8mm), and the live side is energized at full voltage even when not in the Ohms range at all. Tracking and/or flashover is inevitable, and waiting to happen if not already2. The Ohms range measuring circuit does NOT have the benefit of Megohm series resistors; nor can it possibly. I doubt if the circuit can accept more than a few volts applied, or pass tens or hundreds of mA for seconds on end without destruction. So any claim of 'withstands 1000V rms' on these ranges would not pass muster.That the instrument complies with various safety standards to Cat III is irrelevant, as the user would be left with a brick after the event --- but would not need to go to the emergency room. Small consolation that! The makers should at least notify that connecting the probes to a voltage source, as a charged cap, even accidentally while on the Ohms/Diode/Cap ranges could destroy the unit. Instead, this manufacturer continues to harp on certification, and maintains that it is perfectly acceptable to connect a voltage source of up to 1000V! I even suggested to them that they respecify a much lower safe voltage level for the Ohms range. No engineering answers are forthcoming.I am not the sole depository of all knowledge on the subject, and would humbly request to be told any flaws in my arguments.Has any body tested connecting a voltage source while in the Ohms// ranges? Even on some other units? I want to do this under controlled conditions just to see how it goes.

author: bd139
author_rank: Super Contributor
timestamp: November 12, 2017, 02:55:13 pm
content: Thatd need to be a big one to do that sort of damage. Also more than the voltage rating of the device. Could have been a transient on the wrong range. Very low probability.I just bought a 1241C in the end so will tear down for compare.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 12, 2017, 09:37:34 pm
content: Maybe learn to crop? I don't care about what the meter is sitting on.Hard to believe you could get this sort of damage. Did you trace out the circuit for the front end up to the switch contacts? If so, maybe add the paths where you feel things are too tight and post it. Hand drawing is just fine.I would not expect the directly discharging a 150V cap into the meter while it was in the resistance mode would do this.QuoteMost of the manuals I have looked at warn against changing modes with the meter connected and what voltages the resistance function can handle. Strange the manual for this would not cover the basics.Page 5Quotepage 28Quotepage 32,33Quotepage 34QuoteQuoteI only looked at the one Keysight meter and I don't normally use a source with enough energy available to do the sort of damage you appear to show. I am interested in seeing what ever test you come up with along with the results. Be sure to make a video of it for us.I am very impressed that they would offer a replacement especially after someone attempted to repair it.

author: Fungus
author_rank: Super Contributor
timestamp: November 12, 2017, 11:36:18 pm
content: Quote from: bd139 on November 11, 2017, 12:22:52 pmEven though they offered to exchange an old, damaged meter for a new one?

author: bd139
author_rank: Super Contributor
timestamp: November 12, 2017, 11:45:08 pm
content: That's a good point and one I realised earlier. The info provided after I posted that shows that (a) it didn't blow up and kill anyone even if it did die and (b) that damage requires some serious energy to create looking at the pictures and (c) they covered their warranty obligation. Clearly something is amiss here and I'd put it well in user error territory. If it is run within rated limits, there should be no critical failure here. It's properly certified and tested as well.I can only see that a massive transient or high voltage ended up in this. Any high current on the wrong terminals would have blown up somewhere in the front end.Therefore I ordered a U1241C earlier after evaluating the above.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 12, 2017, 11:49:03 pm
content: Quote from: Fungus on November 12, 2017, 11:36:18 pmI doubt many companies would make that offer. Doubt they are swapping it for another B.So if I hook my Gossen to a MOT can I get a free upgrade as long as I claim its a design problem?

author: bd139
author_rank: Super Contributor
timestamp: November 12, 2017, 11:55:32 pm
content: yes it's quite remarkable.I ordered a U1168A lead kit and U1180A thermocouple kit so I'm invested in this one

author: Fungus
author_rank: Super Contributor
timestamp: November 13, 2017, 12:40:20 am
content: Quote from: joeqsmith on November 12, 2017, 09:37:34 pmI'm with joe.Somebody's not 'fessing up to what really happened to it.

author: najrao
author_rank: Regular Contributor
timestamp: November 13, 2017, 07:18:52 am
content: It was easy enough to trace the signal path from hot jack right up to the 'innards. Alas, there is no diagram or schematic. The critical tracking path is at the switch pads, now lost in the photo. More importantly, this short space is left stressed at full voltage at all times of voltage measurement even with the switch open.I tend to believe that a moderate voltage and moisture started the tracking across this 0.8mm gap. Elsewhere, I have seen a picture of how just 12V is enough to track across an inch of phenolic substrate in tropical climate. The burning and charring is progressive over a long time.I have offered to return the unit to the makers, and have requested to be told of their findings.I may be guilty of not reading the instruction manual. But the claim of 1000V withstand is misleading and should have been qualified.I am leaving their offer of replacement to their discretion.Thanks everybody for the support.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 13, 2017, 01:28:16 pm
content: I was not clear with my hand drawn comment. I am suggesting thatYOUtake the time to draw up the handful of parts making up the front end and then adding where you feel there is a clearance problem.Too bad you lost your high res pictures. I guess you could take new ones or was the camera lost as well? Or was it a case where in your attempt yo solve a murder, you destroyed the body before collecting the evidence?It's common for meters to have the full voltage at various parts but normally it is not a direct link. Some like those really cheap Aneng meters people keep going on about, put what little current limit they have after the switch. The switch in that case breaks down and can actually shunt the entire transient. Funny is some people hail that as a good thing. Anyway, I doubt your meter is like that. I assume if the problem was progressive over a long time as you are now suggesting, the meter has been having problems for a long time as well and it went ignored? Seems strange if you are working with KV levels.It sounds now like you may be thinking that the high voltage started this area to have a low resistance and then someone connected a large capacitor charged to 150V and that's when it went. Assuming the 1K resistor is in series with the PTC before it gets to the pad/s in question, I don't buy it as the initial current would be fairly low and if the resistance were low enough to cause the PTC to heat up, the current will fold back even further.Personally, it would have been much better had you documented every detail rather than posting all your feelings about what you felt happened and dragging Keysight through the mud. Asking for people to write you for more details is very strange. At first, I just assumed you were phishing. I've damaged a lot of meters but I always attempt to show the details upfront.

author: najrao
author_rank: Regular Contributor
timestamp: November 14, 2017, 04:34:09 am
content: I am not familiar with photos and sketches to attach, hence the glitches. Now see a diagram.I was NOT phishing, nor seeking any kind of compensation from Keysight. I did not even remotely suggest it to them. Even now, I hv left the replacement offer to their discretion; and will give them the damaged unit unconditionally.My interest is in obtaining a scientific explanation of how this instrument can withstand the claimed 1000V at the input terminal, given the small clearance/s in the internal circuitry. It now appears that the English word "withstand" does not include unmolested survival after the event, and the instrument "withstands" as long it does not blow up in the user's face.Tracking at low level of voltage stress is not very common on fiber glass substrate, but can occur if there was a surface defect or inclusion.Keysight do not suggest that I proceed to test the Ohms input at voltage, as the switch is damaged.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 14, 2017, 12:59:30 pm
content: Thanks for taking the time to post this. On the backside of the contacts that burned, I wonder if there is another series resistor and then the clamp, or just the clamp? This will be there to provide some protection for the driver. Could you please take a close look at the two GDTs and provide any markings that are on them?****************!!! Please recheck the PTCs !!! Are these really 22 ohms? If they indeed measure this low, could you please cut away any shrink tube and provide any markings on these as well. Thanks.****************Maybe we can make some sort of model of the meter for the fun of it.I have started to life cycle test a few meters function switches. As the parts wear, the metal particulates could very well end up in the area between the pads.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 14, 2017, 01:40:57 pm
content: Besides a few other Keysight handheld multimeters, I also have a U1241B and have used it many times for up to 1000 V DC, without any Problems at all.It is rated CAT III 1000V / CAT IV 600V and has fulfilled all my needs for this handheld.Something awefull must have happened to yours, far outside the normal range of operations and it should not be Keysight's responsibility to replace it.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 14, 2017, 02:10:58 pm
content: It's more than 0.8mm, like 1.1-1.2mm:

author: Fungus
author_rank: Super Contributor
timestamp: November 14, 2017, 03:03:57 pm
content: Quote from: joeqsmith on November 14, 2017, 12:59:30 pmIt's weird that there's no resistor ladder as part of the input protection. I'm guessing there's one on the other side of the PCB.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 14, 2017, 03:22:25 pm
content: Quote from: Fungus on November 14, 2017, 03:03:57 pmNo, nothing. Also, the left resistor in OP's drawing is only 275 Ohm in my meter.

author: Fungus
author_rank: Super Contributor
timestamp: November 14, 2017, 03:33:05 pm
content: Quote from: PA0PBZ on November 14, 2017, 03:22:25 pmJoes reverse engineering (in his video) shows similar values.Low values allow the PTC to heat up nicely. The real protection for the rest of the meter should be in the resistor ladder after the PTC.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 14, 2017, 03:43:29 pm
content: Quote from: Fungus on November 14, 2017, 03:33:05 pmFound it: Other side of the board,afterthe range switch, 5 x 2M.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 14, 2017, 04:02:17 pm
content: QuoteThe PTCs I measured were not even close to OP's 22 ohms. I have never seen a PTC with this low a value used in a handheld meter.QuoteThere are two legs. One is an input, the other an output. While the input side has a very high input impedance and will commonly have some sort of high voltage resistor or several smaller parts, the output side is a low impedance path.I have seen meters with nothing for protection on the input side. They count on the high impedance to protect the meter. The drive side is a little more tricky but it's not rocket science. The protection schemes are fairly well known.Dave's vapormeter is a little odd because it could drive at such a high output voltage. If you caught those early videos I made of the meter, you may remember me adding a TVS to that circuit along with changing to a CMOS part to try and increase the margins. It seemed Dave had made some comments about the meter doing as good as the 87V which only ever fails on my test jigs and that the cert was good enough. Later he posted about this circuit using a TVS. It will be interesting to see the released version of that meter, but that's for another topic.Back to OP's meter. As I mentioned, there will normally be a low voltage clamp on the backside of the switch. There will be enough current that will cause the PTC to heat. Even if the contacts shorted, all that should happen is the PTC would heat up. But with a 22 ohm part, I wonder what the thermal time constant is....

author: Fungus
author_rank: Super Contributor
timestamp: November 14, 2017, 04:34:39 pm
content: Quote from: joeqsmith on November 14, 2017, 04:02:17 pmWe were talking about "the left resistor".Quote from: PA0PBZ on November 14, 2017, 03:22:25 pm

author: Fungus
author_rank: Super Contributor
timestamp: November 14, 2017, 04:47:23 pm
content: Quote from: PA0PBZ on November 14, 2017, 03:43:29 pmTold you so!

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 14, 2017, 05:12:07 pm
content: Quote from: joeqsmith on November 14, 2017, 04:02:17 pmThe PTCs in my meter are around 800 Ohm.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 14, 2017, 06:29:27 pm
content: Quote from: PA0PBZ on November 14, 2017, 05:12:07 pmI would buy that. If OP measured them correctly, it will be very interesting to see how they are marked. If they really are 22 ohms and marked this way, the next question is did the factory do this, or was the meter damaged at some point (I've cracked a few PTCs) and someone with exception intelligence wanted to save a few bucks and play technician? Still it does not explain why it would arch across the switch in the first place, unless if the meter was previously damaged, maybe it had arced across before...I like a good murder mystery.

author: najrao
author_rank: Regular Contributor
timestamp: November 15, 2017, 02:52:19 am
content: I have rechecked the PTC's to find that 22 ohms is wrong: they show 680 and 710. It was my fault entirely. The 'fusible' resistors are 1k on the Volts path and 100 ohms on the Ohms line. Markings are hidden below shrink sleeves. Keysight want all evidence retained, so I will not cut open.The switch track clearance may have been a bit more than 0.8mm; there was much charring and I did not use a vernier.There are no other protection components on the board.Once again, on the Volts range, there is no issue other than the fact that the Ohms switch remains hot on one side, stressing the small clearance. The 2×5M resistors should definitely be enough to survive 1000V, even with a crest factor of 3.On the Ohms range, current is limited essentially by the 100 ohm protection resistor and the PTC, unless the measurement circuit is able to take up much of the applied voltage. If it clamps, there will be too much current and the PTC should heat up and go high, but not for a time. The current can reach several hundred milliamps initially and last seconds; can the measurement circuit hold its clamping action?On the other hand, nearly all the impressed voltage will reach the measurement circuit if there is no clamping. This seems almost impossible, given the track clearnce of 0.25mm.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 15, 2017, 04:03:06 am
content: Thanks for rechecking the PTCs. There for sure are other protection parts on the board. The meter would not survive without them. These will be on the backside of the switch you damaged. You would need to trace it out. This clamp will hold that side of the switch fairly low. The remaining voltage will be across the PTC and resistor as you suggest (at a KV, basically all of it).Yes the second clamp I am referring to should be designed to handle the current for the length of the thermal time constant of the PTC. Likewise, the resistor should handle the full voltage as well for that short time. My guess, based on the pads being damaged the way they were is the parts making up that secondary clamp would be visibly damaged. I'm actually surprised that with the pads repaired that the meter would still read resistance, capacitance or diode check. If the clamp failed short, the parts down stream may survive but the meter would not function. This is the best case failure. lol. I've blown them open which never works out well for the parts down stream from the clamp that was supposed to protect them. Normally this is after the PTC arcs over and I have seen a lot of piss ant PTCs come apart..If you cut away the jacket, you may be able to find the datasheet for the PTCs which will have the curves for the time constant. You could calculate if it was a problem easy enough. It still would not answer why the switch had such a violent ending.Of course, if the input side GDT fires it has nothing to do with the drive side. So the switch sees the spike until the drive side tube fires. But these are transient conditions. It sounds like you are claiming that you are not running anywhere near where the tubes would even turn on. Really strange..It will be interesting to see if Keysight provides any further guess to what went wrong..

author: najrao
author_rank: Regular Contributor
timestamp: November 15, 2017, 05:45:17 am
content: Excellent, Joeqsmith. At last someone got to the points I made in the first place. No Cats at all!Agree there has to be a low side clamp, but tracing what and where is well nigh impossible, there are no discretes as such energy clamp just has to be one. Applies to both sides of the board. But then one may be hidden behind the display, for example.I am equally surprised that the failure event did not destroy the clamp or the measuring circuit ahead. Indeed the PTC and fusible resistor look like they never saw any duty. Even now, the Ohms, Diode and Cap work as nothing happened at all. Not calibration checked, no, but good for all purposes.PTC's can heat up very fast once they get going in a maintained current situation, and simple thermal time constant calculation is all but absurd. Wild guess is best.My repair solution is not all it should be. I have just wired a reed relay to points leading to the obliterated switch, and operate it by a Nd-Fe magnet from outside the case. The reed contacts are just 0.5 spaced, and it is easy to forget withdrawing the magnet before a voltage measurement. I would not recommend it.Keysight appear to have gone slow after they made the replacement offer. But I will give the failed unit even if they don't replace. There are logistics difficulties in shipping.It is possible that other models by Keysight have the same issues, would their owners check? Not really difficult, you hv to take out the board and examine visually.

author: HKJ
author_rank: Super Contributor
timestamp: November 15, 2017, 08:06:56 am
content: As far as I understand the input protection:The MOV is there to protect the switch and high impedance path.There are usual two or 3 path from the input: A 10Mohm, often a 1Mohm and a low impedance for driving ohm/capacity (Usual in the 0.5-2kohm range).After the switch are low voltage clamps on all the paths, the low impedance clamp must handle fairly high current until the PTC heats up.

author: najrao
author_rank: Regular Contributor
timestamp: November 15, 2017, 09:41:57 am
content: Thanks, HKJ and Joeqsmith. Your input drove me to check again, more carefully this time. I have to eat my own words again.Bingo! There are TWO discrete diodes, may be zeners, and one of them clamps positive voltage to unknown level to ground, negative to a diode drop. This diode has a marking "457A" as far as I can read by magnifying glass. Some in situ testing shows clamping action at 3V or so for a modest 8mA through the Ohms input. A negative current of 9mA clamped at -- 0.73V, as expected. All voltages referenced to COM input terminal.The other discrete diode is marked K7, but looks identical. It's cathode ends up somewhere deep, so I cd not trace further.How Keyswitch protects the Ohms range is now clear. It is unlikely that they have not engineered the circuit for the PTC heating up time.This resolves the track spacing issue as well: the potential on the cold end of the PTC can never exceed the clamp level, positive or negative, and so the 0.25 or so is still good enough.But it does not absolve them of the poor switch clearance, 0.8mm.We have got somewhere after All! Thanks.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 15, 2017, 01:11:42 pm
content: I was going to point you to a high res picture that showed what appeared to be the secondary clamps. Glad you took the time to find them. I suspect the one is a TVS. For me, we are no further along. You have a better idea of how the circuit is designed but it still does not answer what happened to the contacts. You got rid of the body before presenting the case and asking to help solve this murder.If I owned one of these meters, I would have no problems putting some DC into the input and seeing where it clamps and the dialing it back and letting it sit for some extended period. Obviously if the switch breaks down we have a problem.Dave did a review not too long ago using a insulation tester on a cheap ANENG meter. This meter has the input, through a fuse into a switch with the other side of the switch to a TVS that returns to the common. In this case, those contacts would arc (I would need to check at what level). But it is not a big surprise. The pads I am sure had some damage from the arcs but that was a fairly low energy transient. The were not destroyed as you suggest yours were. When I test them, I use up to around 20 Joules. The generator has a two ohm source impedance. This has not been enough to cause the level of damage you describe. At most, maybe some discrete components come apart. It appears yours was hit with something much larger. High res pictures of the damaged area may have helped answer that.I at times will run what I am calling a half cycle test on them. It's nothing I keep data on. This waveform simulates a half AC cycle of something a little more reasonable energy wise. It also uses a two ohm source. The generator can be applied to a quality meter and nothing will happen as the voltage is lower than what the meters are rated for. However, it works with the higher voltage generator. Basically, the idea was to simulate a transient on an AC line but still keep the energy to something I personally feel safe working around. If the high voltage transient causes something inside the meter to breakdown, the low voltage generator then feeds the fire. It's nothing like what could happen in real life but does give me some idea how a meter may behave. Anyway, this setup can cause the sorts of damage you describe. And it is very typical for the switch to provide spark gap and needed copper.So if you were looking at an AC bus and something like this came through things like this can happen. This is where the CAT rating comes in. It's a safety standard with as I understand it is there to help prevent damage to the operator. From talking with meter manufacture's some design their products to survive the transients called out by these standards, other do not. I only ran the one Keysight meter. It is no longer with us.... You may have felt you were looking at a 440 AC bus which the meter was rated for.... Again, hard to say.

author: najrao
author_rank: Regular Contributor
timestamp: November 16, 2017, 03:49:21 am
content: Thanks, Joeqsmith.I continue to believe that the clamp is just a zener and not a high voltage TVS, because it has to clamp at very low voltage which is unusual for TVS. At anything over a few volts, it will need to dissipate, and survive after, several watts. Remember the applied voltage may be any value below 1000V; at 100V, there is a current of just under 100mA, and the PTC would take longer to cut this down. At some even lower voltage, the current will get to be too small to heat the PTC, yet be large enough to cook the zener, unless it clamps at just a few volts. The zener is 500mW at best and has no lead conducted cooling. More, it has done its job through the catastrophe when the switch arced over, and is none the worse for it after. I would not hesitate to test it all out, but Keysight want to investigate further in their own way and would need the evidence in tact.Agree that we still don't know what caused the switch to arc over in the first place.There is data on 1N4574, but the marking is 457A. 4574 is a special temperature compensated reference zener at 6.4V.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 16, 2017, 04:54:58 am
content: TVS's are not at all limited to high voltages. Normally people don't measure the output resistance of their active power supplies, but even if they do the PTC is there to save you from yourself. Check the curves for some PTCs and see how quick something like the one you have will switch. Sure, the peak power can be quite high but that is not a steady state condition.The majority of the meters I have looked at will use two back to back SOT23 transistors or diodes to form the secondary high speed clamp. Cheap, low leakage. Off the top of my head, the BAV99, BAV199 and 3904s seem to be common. They can handle a fair amount of current for a short time.I doubt very much in your case that the PTC did anything but act as a resistor. A pulse large enough to arc over and cause this sort of damage, I would guess was all over before the PTC budged. However, you are correct that for those of us, (I am talking about myself) who seem to have a strange desire to measure the resistance of their active power supplies just because we want to see how robust these meters are (I'll give you this is not a typical case), I really haven't seen much of a problem with the clamps or switches in the sub KV levels. If you watch any of my videos, you will know it is not for a lack of trying. The typical life of a meter in my hands is measured in days..To be clear, I am not suggesting that your meter doesn't have a problem but I really have no way to verify it without checking it first hand. If I had one, I would gladly attempt to smoke it just for you.

author: najrao
author_rank: Regular Contributor
timestamp: November 16, 2017, 05:30:10 am
content: Thanks, Joeqsmith.I continue to believe that the clamp is just a zener and not a high voltage TVS, because it has to clamp at very low voltage which is unusual for TVS. At anything over a few volts, it will need to dissipate, and survive after, several watts. Remember the applied voltage may be any value below 1000V; at 100V, there is a current of just under 100mA, and the PTC would take longer to cut this down. At some even lower voltage, the current will get to be too small to heat the PTC, yet be large enough to cook the zener, unless it clamps at just a few volts. The zener is 500mW at best and has no lead conducted cooling. More, it has done its job through the catastrophe when the switch arced over, and is none the worse for it after. I would not hesitate to test it all out, but Keysight want to investigate further in their own way and would need the evidence in tact.Agree that we still don't know what caused the switch to arc over in the first place.There is data on 1N4574, but the marking is 457A. 4574 is a special temperature compensated reference zener at 6.4V.Keyswitch hv just said that they will go ahead with the further investigation and are arranging to take it back. Let us wait.

author: najrao
author_rank: Regular Contributor
timestamp: December 15, 2017, 11:46:46 am
content: True to their word, Keysight has sent me a replacement. Very generous of them.In turn, I have given them the old unit to investigate. We need to wait and watch for any report from them.I will get back here if/when they do send their findings.