Saftey of high voltage probe (PCBs?) old wiring? - Page 1
Forum section: electronics / testgear

author: ftransform
author_rank: Frequent Contributor
timestamp: May 14, 2013, 04:43:41 am
content: So I bought a high voltage probe on ebay, RCA brand, and I noticed that when I took it apart it smelled kinda funky. I believe it is made in the 1950's but there is no model number or anything like that.I don't think that PCB's smell bad, but the bad smell made me feel kinda anxious. How safe are plastics from the 1950's? How about the wire on this probe?Should I just avoid this 1950's shit?This probe has some soft plastics in it, as I understand it hard plastics are OK but soft plastics = bad. Should I just take the 1 gigaohm resistor and toss it?My heathkit HV probe has no soft plastics in it..

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 04:47:21 am
content: Electronics that old often have interesting... fragrances. That said, it's a high voltage probe. I don't think I'd trust an HV probe from the 50s as far as I could throw it with a broken arm.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 14, 2013, 05:01:46 am
content: Quote from: c4757p on May 14, 2013, 04:47:21 amwhy, its just a piece of plastic with a gigaohm resistor in it.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 14, 2013, 05:05:46 am
content: For ththird questionYes, you should avoid 1950-1960-1970 shit

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 05:12:33 am
content: How do you know it's still a gigaohm resistor? Do you have something that can measure that high? And how do you know it won't collapse under high voltage? I'm sure it's well past its life expectancy.

author: pickle9000
author_rank: Super Contributor
timestamp: May 14, 2013, 05:17:59 am
content: This is one of those things where you are taking you life in your hands. Even if (and that's a big if) the plastics and such lasted what is it's history, how was it stored, abused, used or even just dropped. Where is the confidence that it's safe?

author: ftransform
author_rank: Frequent Contributor
timestamp: May 14, 2013, 05:20:43 am
content: I don't understand how the plastic can become conductive over time.And yes I have a electrometer I can measure it with.I am more worried about the chemical off gassing then anything else.And it is a carbon wound resistor.. how can it collapse under high voltage?I am interested in these phenomena.Seriously, I am not trying to be snotty, but what could go wrong?then again my "pasco" brand electrometer is probably not the most trust worthy device lolBut plugging it into any source should be enough to see if its dividing correctly.I see it like this. The probe is a foot long, it has a 6 inch long resistor in it.SO long the surface of the probe does not become conductive or the carbon resistor a short, its ok.The resistor inside looks like a carbon film covered with a clear epoxy.I don't imagine any kind of creepage would be a problem because its not exposed to high voltages for any long duration of time. It just seems like something that I can visually inspect to be confident in.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 14, 2013, 05:55:59 am
content: Quote from: pickle9000 on May 14, 2013, 05:17:59 amAnd no, I would not trust my life to even a fluke probe. If the circuit is that high energy im going to fix the probe in place while its off, measure it hot, then turn it off and discharge it.This is more along the lines of low current high voltage, I am not really interested in lethal stuff.

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 05:56:32 am
content: Old resistors drift all over the place. Carbon composition, for example, can change value permanently under voltage stress. (I know you said it's a carbon film.) No, I don't understand the mechanism, but I've not seen any type of resistor that doesn't drift, sometimes significantly, over decades. I have pulled 100k "precision wirewound" (yup, 100k worth of wire) measuring under 10k from old voltmeters. If you have any reason to trust it, then it's up to you, but for me all HV equipment is guilty until proven innocent.

author: c4757p
author_rank: Super Contributor
timestamp: May 14, 2013, 05:59:40 am
content: Even "low current" HV can be dangerous. What if it charges a capacitance? Where is the current limit when you bridge and discharge that?

author: edavid
author_rank: Super Contributor
timestamp: May 14, 2013, 07:52:31 pm
content: I really doubt it's from the 50s... more likely the 70s or 80s (although if it's marked RCA instead of Viz, that sets an earlier date).What are you going to use it for? It was probably designed for 25kV TV sets, so if you use it on 1000V circuits, there's a lot of safety margin.

author: Fraser
author_rank: Super Contributor
timestamp: May 14, 2013, 08:05:12 pm
content: To answer the question on plastics ageing.....Plastic or Bakalite is a combination of materials that when manufactured offered a dielectric strength adequate for the task to which they have been applied. Over many years plastics change and can lose qualities that they once possessed. If the plastic or Bakalite is even slightly porous due to age, it will no longer be an EHT insulator and could present a path to you body from an EHT source, or just poor reading accuracy combined with corona discharge. Moisture uptake in insulators can be a real problem hence why glazed ceramics or glass are used on railway supplies.Any PCB material that is in the EHT circuit would be highly suspect after a few decades as moisture content is once again an issue.As a simple rule to follow, if an EHT circuit has the potential to harm you rather than just cause a sting, you should use modern EHT divider probes of known quality and performance. If the voltage would just sting a little, use the EHT divider that you have but accept that it could give erroneous readings due to corona discharge or calibration drift. I have a safe method of testing such probes up to 45kV as I own a large insulation leakage tester but that costs many times the purchase price of a new EHT probe.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 14, 2013, 09:52:37 pm
content: Plastic is usually maid of carbonium chainSo it's not difficult to imagine that under stress (burn, radiation) it can becom conductive

author: vk6zgo
author_rank: Super Contributor
timestamp: May 15, 2013, 02:03:56 am
content: Quote from: ftransform on May 14, 2013, 04:43:41 amThe "hard plastics OK,soft plastics,bad" rule is not very useful.PTFE is a "soft" plastic,as is Nylon,& both have good insulating properties (although Nylon isn't too happy with RF).All the EHT probes I have seen from the 1950s/60s use a large conical shaped moulding made of hard Polystyrene,with a metal tip on one end,a high voltage 1000M Ohm resistor inside this cone,which then connects to the lead to the meter.Hard Polystyrene has good insulating properties,& maintains them over a very long time,if it is not exposedto ultra-violet light,weathering,etc.I've never seen one of the high value resistors with anything wrong with it except physical damage.In service,these probes may only clock up a few hours a year,so they are not in service continually,as is usually the case with resistors changing in value.Another point is that the probe may not have been designed for use with the kind of meter you have.For instance,at my old work,we had some Electrostatic voltmeters which used very similar probes,but may have had very different input impedances to a modern DMM.If it is useable,I would sooner trust an old unit made by a reputable company in Australia,the USA,the UK.or Germany,etc,than some crud modern thing made in the PRC.These were not cheap units originally,& would,in most cases been "babied" throughout their service life with the original company.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 15, 2013, 07:11:13 am
content: Hmm, this is where a high voltage calibrator would come in handy. I need to make sure my probe is made of polystyrene, then I wont have to worry about leakage there.The thing that concerns me now is the coating on the internal resistor. I think it is glass but it demands another look....By hard and soft plastic I mean the possibility of it having PCB (the cancer chemical) as a plasticizer.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 15, 2013, 09:43:34 am
content: you fool trash this thing, we're speaking about your life idiot!

author: amyk
author_rank: Super Contributor
timestamp: May 15, 2013, 10:42:53 am
content: Is it the smell of phenolic resin? Bakelite will smell like that.Polystyrene doesn't have any smell, although styrene smells slightly sweet.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 16, 2013, 08:17:07 am
content: Quote from: ddavidebor on May 15, 2013, 09:43:34 amrelax bro, plastics make it possible

author: ftransform
author_rank: Frequent Contributor
timestamp: May 16, 2013, 08:19:29 am
content: Quote from: amyk on May 15, 2013, 10:42:53 amNah, it smelled like oil. I said smelled because I tried to wash it in soap but getting the stench out was impossible, so I just took the resistor out of it and tossed it. I read using solvents on these probes is ill advised.I will stick to my heathkit brand new looking polystyrene probe.http://www.utm.edu/staff/leeb/Probe.pdfI have a different value resistor though.

author: Fraser
author_rank: Super Contributor
timestamp: May 16, 2013, 11:14:14 am
content: I have that Heathkit probe. It was designed for their Valve Voltmeter and as you know the Voltmeter has a non standard input impedance.As to it being a new probe, do you mean new old stock as this is an ancient probe and possibly has the same issues as the one we have been discussing. My modern Fluke and Testec EHT probes are of a far higher build quality. As I said before, if the EHT is a danger to you, use only a modern quality EHT probe as your life is worth far more than what they cost .I am attaching the manual for the EHT and HT probes that I am currently using. I was lucky enough to buy these at very reasonable prices so have a good EHT measuring capability as a result.The Testec manual gives guidance on probe care which may be of interest to you.As a final comment..... that Heathkit probe that I have is destined for the bin as it is a remnant of my college days 30 years ago, when I couldn't afford a decent EHT probe. I used that probe and the associated VVM to check for the presence of 10KV on monochrome TV CRTs....non lethal but it stings if you get in its way !

author: vk6zgo
author_rank: Super Contributor
timestamp: May 17, 2013, 12:20:30 am
content: Having had the opportunity to examine a 1980s/90s Fluke EHT probe side by side with a 1959 AWA EHT probe,the type of construction & build quality was virtually identical,but the resistor was a bit longer in the AWA.This was unfortunate,as when the Fluke resistor failed,my hope of a "quick fix" by stealing the other resistor was foiled.OK,there would have been a few % error,but who cares in the real world?Are you going to hang this thing off a multi-KV mains distribution system?If not,for most of the voltages you will deal with,an old EHT probe in good condition will more than handleanything you throw at it.You have eyes!--Examine the insulation,look for stress cracks & the like.The design of these things is a mature technology,& has pretty much standardised over the years.

author: Fraser
author_rank: Super Contributor
timestamp: May 17, 2013, 06:09:49 pm
content: I have to agree with VK6ZGO,Most EHT systems that I have worked on could deliver a nasty 'bite' and possibly a pinhole burn to the skin but are unlikely to do much more unless you wear a pacemaker. As has been said this is a mature technology that relies upon the dielectric strength of the insulation materials used combined with the distance that the the EHT would have to travel in order to reach your hand. This is one of the reasons for the disc shaped fins on some models (to increase distance of travel for a surface arc). Common sense rules here but please be aware that some common EHT sources such as those found in Microwave Ovens and modern 40kV CDI car ignition systems can kill the unwary. My EHT probes are supplied with an alternative 'hook' tip that enabled the probe to be hung off of the target test point and so a hands free EHT test may be performed in safetySuch a yip could be DIY made using a small brass picture hook soldered to a brass nut or screw, depending upon the tip fitting.remember to always attach the 0v cable to the 0v line of the EHT circuit !