Fluke 8060A Dual fuses - Bad Design? - Page 1
Forum section: electronics / testgear

author: warf135
author_rank: Contributor
timestamp: March 12, 2014, 10:37:06 pm
content: Hello All,I have just bought a Fluke 8060A, which seems like a decent meter, although I'm a bit annoyed at one part of the design of the meter.For the Amps protection, it has 2 fuses in series. A 2 amp small glass fuse in the battery compartment and a 3a/600v proper HRC 'Buss Fuse' inside the meter.The little glass fuse had obviously been replaced before I bought the meter, but not the internal HRC fuse... It annoys me that I had to break the (admittedly out of date) calibration seal to be able to replace the fuse.I have never seen anything that uses 2 different fuses in series ever before. Surely using a little glass fuse in series with the HRC 'proper' fuse totally negates the HRC fuse? or am I missing something? The glass fuse is 2a and the HRC fuse is 3a so designed for the glass fuse to blow first?Can anyone explain why they have done it this way? Cos I cant understand it!Cheers,Norm (aka Warf)ps i can upload full schematic if required

author: c4757p
author_rank: Super Contributor
timestamp: March 12, 2014, 10:46:19 pm
content: Yes, the glass fuse will blow first, so you can replace a less expensive fuse. If a high-energy current has to be interrupted and the glass fuse fails to interrupt, the HRC fuse will then pop.

author: warf135
author_rank: Contributor
timestamp: March 12, 2014, 10:56:44 pm
content: I did nearly fall off my chair when I saw the price of the HRC fuseI assumed that the 2a glass fuse would just blow first all the time. Didn't realize that it might not blow...

author: tom66
author_rank: Super Contributor
timestamp: March 12, 2014, 11:03:38 pm
content: Oh, the glass fuse will blow. It will explode, but the arc will remain. That's what the HRC fuse is for.Quite a clever design, wouldn't expect to see it nowadays, given they'd like to sell more fuses.

author: warf135
author_rank: Contributor
timestamp: March 12, 2014, 11:29:23 pm
content: Ah, interesting. Though I would still of liked a way to change the HRC fuse without having to take the whole meter apart! (cant please some people lol)I notice in the manual that the 8060A was first made/sold in 1982 and my one was made after 1993, I don't know how long after 1993 they continued making it, but even now in 2014 it seems to be a very good meter.I'm not sure if I like the side selector switches over a rotary selector, but its growing on me

author: Robomeds
author_rank: Frequent Contributor
timestamp: March 13, 2014, 03:11:27 am
content: The Fluke 8025A and 8025B use the same setup. However, my Fluke 27 (nearly identical to the 8025s) does not. I have no idea if this is considered safe but it is nicer than replacing $10 fuses because you had a brain gas release.

author: c4757p
author_rank: Super Contributor
timestamp: March 13, 2014, 03:21:58 am
content: Quote from: warf135 on March 12, 2014, 11:29:23 pmPutting it deep inside decreases the chance that it'll do something evil to you if for some reason it actually does fail catastrophically, and discourages people from just slapping in any old glass fuse.They did the same thing in the 8050A (bench meter). You can change the glass fuse from the front panel, but if you want to change the main fuse you have to break the cal seal.Actually - another reason: If it's suffered such an event as to arc over the glass fuse, it probably needs a recal anyway...

author: warf135
author_rank: Contributor
timestamp: March 13, 2014, 06:00:04 am
content: Quote from: Robomeds on March 13, 2014, 03:11:27 amThose brain gas releases happen quite regularly in my shed!Quote from: c4757p on March 13, 2014, 03:21:58 amGood Point! I've not tested the amps range since replacing the fuse... All voltage ranges seem ok (according to my cheap voltage reference). I have a Quark PMM440 which can generate upto 100 mA, so I'll test it when I get chance.I don't really need it to be super accurate as I don't currently work on anything that needs precision, so recal not really needed ATM, as long as it gives similar readings to my other meters that will do for now.

author: idpromnut
author_rank: Unknown
timestamp: March 13, 2014, 12:38:18 pm
content: Another thing to keep in mind is the rated current, which I don't think is over the value for the glass fuse. So in theory, in "normal everyday use", you shouldn't be knowingly subjecting the 8060 to a current that will pop the HRC fuse. And if you unknowingly do so, the HRC fuse will do it's job and save your bacon(at which point you would need to re-cal the meter anyways, so popping the hood to replace the HRC fuse isn't a big deal).

author: Wytnucls
author_rank: Super Contributor
timestamp: March 13, 2014, 01:05:11 pm
content: To make sure that the glass fuse blows before the HRC one, check that it has a proper fast blow rating (FF) and not a medium or slow blow.

author: warf135
author_rank: Contributor
timestamp: March 13, 2014, 04:40:10 pm
content: Thankyou everyone for your replies, I can see now that its actually GOOD design using the 2 different fuses in series

author: drtaylor
author_rank: Regular Contributor
timestamp: December 30, 2015, 07:06:03 am
content: A very good design that saves the user from having the meter blow up in his hand. In reality, this design hadn't been used in the handheld meters due to space. The 2A fuse inside the battery door was all the basic case design had room for. So due to a decree that modifying the basic 8020 shape to accommodate the 8060A would be kept to a minimum, I put the HRC fuse inside. During normal operation, the 2A fuse would be all that should blow. The 3A is primarily there to protect the user. If enough energy is present to arc the 2A fuse, the 3A opens without the device smoking and burning. We did some early tests with high energy inputs into the current pins, and one time the bridge rectifier exploded and a flame shot through the plastic case. So the HRC fuse was added. The 2A fuse protects the shunts, the 3A fuse protects the user. No guarantee that the circuitry will survive given enough instantaneous power.

author: jitter
author_rank: Frequent Contributor
timestamp: December 30, 2015, 08:31:16 am
content: Quote from: tom66 on March 12, 2014, 11:03:38 pmYes, a grossly overloaded glass fuse (think: trying to measure mains voltage with the leads still connected to the amp busses of a cheap ass multimeter), will indeed shatter...That's why I wonder if that glass fuse is what it actually left the factory with. I would have expected a similarly acting but stronger type, i.e. ceramic instead of glass.

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2015, 09:54:52 am
content: Quote from: jitter on December 30, 2015, 08:31:16 amYou overlook that member drtaylor designed the 8060 and his reasons for this 2 fuse design are explained above.The thread he started to record his experiences for us:https://www.eevblog.com/forum/testgear/old-fluke-multimeters/

author: jitter
author_rank: Frequent Contributor
timestamp: December 31, 2015, 07:09:13 am
content: Quote from: tautech on December 30, 2015, 09:54:52 amI may have overlooked that he designed the meter, but the reason for two fuses in series was already clear to me.I found the manual of the 8060 and it confirms that F2 was indeed a glass rather than a ceramic fuse of the same size and charachteristics.I just do not understand the use of aglassfuse, regardless of the HRC fuse in series with it, I would have epxected a fuse strong enough not to mechanically shatter in any overcurrent situation.

author: Tim F
author_rank: Regular Contributor
timestamp: December 31, 2015, 12:13:01 pm
content: I always wondered this as well, but I assume from drtaylors comments about testing the design that the HRC fuse opens the circuit before the glass fuse goes boom in a big way.

author: Electro Detective
author_rank: Super Contributor
timestamp: April 30, 2017, 02:09:20 am
content: Post RevivalI have been meaning (for years lol) to replace the small fast blow 2 amp glass fuse with 2 amp generic (cheap) ceramic fast blow in my 8060A handheld and 8050A bench meter, and some decent Taiwan made meters from back in the day, and a couple of analogue oscilloscope etcOne benefit I see is perhaps a better higher rated tighter spec fuse (?) and less mopping up to do if the glass fuse shatters or shrapnels in the event of a mancave bench test oopsie, or a quietly returned loanerAny thoughts before I shell out some dollars on affordable/cheap ceramics, performing a crash test beforehand on a couple of fuses in a batch/packet to confirm they do blow the same as the glass ones, and not blow at the same fault current as the internal 3 amp HRC on a mild hiccup.