Beginners: Measuring high voltages in a safer way

[Andrewausfa]

You have to remember that I am a beginner and still very nervous of sticking things in there when it's plugged in.

That's a good concern not a bad one, which is why I suggested clips so you can stand back and watch your meter in safety I've done you a photo or two in between making dinner.

Insulated croc clips. your meter prods fit in the end.

Measuring voltage on smoothing can, positive clip is on one of the cans tags, negative to chassis.

Next step is to switch off, remove plug from mains, set up the test leads for the next measurement, plug in, switch on, take the measurement. Switch off, remove plug from mains. You don't touch the leads at all while power is applied.

Andrew

[Terrykc]

I appreciate concerns about touching (high) voltage points while a set is switched on but insulated crocodile clips are far from ideal for the purpose as the physical size of the exposed metal part makes it very difficult not to create a short to nearby tags and valveholder pins, etc.

The probes supplied with many meters are like these:

Again, there is a lot of metal on these and, of course, they require two hands to hold them in place, so the black one is best replaced with a crocodile clip. (Note, as the DMM is similar, I've highlighted the 1000V DC range ...*)

The best thing to do with such a probe is fit a piece of snug fitting sleeving over the metal - a piece of insulation from a bit of wire or flex often works well - so that only the very end of the tip is exposed.

Remembering the left hand rule - assuming you are right handed - clip the crocodile clip securely on the chassis, start off on the 1000V DC range with your left hand safely in your pocket and probe around with relative impunity - your right hand should never go closer to anything 'dangerous' than the length of the probe which can be up to 100mm or so.

However, if you can't pluck up that much courage (yet) - don't despair: a helping hand like this will resolve the problem:

Probe_Clip.jpg (23.73 KiB) Viewed 3332 times

This neat clip is easily secured to the point you want to measure before you turn the set on. It is secure yet small enough not to cause problems if you fit it carefully.

Personally, I kind the best of both worlds is a similar telescopic probe about 100mm or so long (but I couldn't find a picture in a quick search). This is long enough to poke around in most environments yet can instantly be clipped on to any point requiring monitoring for more than a second or two. If you can find one (or a pair) they will pay dividends in the future.

* The golden rule is to always start off on the highest range, then work downwards if necessary.

[Terrykc]

... You have to remember that I am a beginner and still very nervous of sticking things in there when it's plugged in ...

A highly understandable reaction. On the other hand, given the pair of croc clip leads shown in your pictures, I would have similar feelings also ...!

Consider this: if you switch on a live chassis set in its operational mode - neatly encased in an insulated box with the back fitted - you will quite happily grasp an insulated knob which places your fingers within 20mm or less of a potentially live spindle but you are very wary about touching the inside of the same set with a probe that keeps you fingers over 100mm away from the same live parts ...

Sounds illogical, doesn't it? But I can sympathise ...

I was always quite happy handling live chassis sets - indeed, when I first started work, I didn't have much choice! Most sets we serviced had reversible two-pin connectors, so it was a 50-50 chance whether the chassis was 'live' anyway ...

However, shortly after that, work started on expanding the building to provide a much extended shop area and a new, large workshop. Eventually the building was almost complete except for a hole at the far end of the concrete shop floor where the electricity service cable emerged from the adjacent building and to which our new electrical feed needed to be connected ...

The man from the electricity board turned up and laid a rubber mat over the edge of the hole, then happily sat down on it and commenced to splice our new cable into the live service cable. Everybody who witnesed this retreated to a safe distance and said "Sd that for a game of soldiers!", yet he was doing nothing more than we did all day long ...

So yes, I quite sympathise with your feelings!

Why not try one of these - or similar?



"These probe clips feature a spring loaded wire hooked probe which retracts into the probe moulding. The probe material is copper and the housing is ABS."

£1.99 from Maplin. http://www.maplin.co.uk/large-probe-clips-46648

It will clip onto the points you want to get measurements from with the set switched off and, when you manage to conquer your fears, should also be useful for "sticking things in there when it's plugged in".

Mind you, I still don't fancy sitting down on one of those rubber mats ...!

[unclereggie]

Thanks for the photos Andy and the advice there Terry. I shall get one of the retracting clips then experiment a little (probably following up in the thread about the MURPHY U198H set)

[unclereggie]

Holy-moly! I can't see myself wearing one of those suits as I tinker about with my radios...

[Michael Watterson]

Fortuntely it's hazard we're very unlikely to encounter in our dealings with vintage kit.

A spanner or screwdriver dropped on 12V or 6V Lead acid terminals can kill. Vibrator fed car Radios and table models (I have a 6V KB running on a discarded ex UPS hacked 14AH 12V Gel cell).

Also authentically run very early models may have 4V or 6V LT from Lead Acid and a scary tray of 140V using 70 off 2V lead Acid cells.

NiMH / NiCd packs can set stuff on fire and give a dangerous flash (Military sets often use 24V 4AH)

None the same as shorting incoming AC mains, but life threatening.

Shorting one coin cell is safe. Shorting several hundred in a small box in series and paralleled with diodes can give thermal runaway and fire + explosion. Experience. I now fit fuses on battery packs.

[Michael Watterson]

An Arc flash at worst can leave you blind and a best you'll wake up that night in terrible pain. Assuming similar characteristics to high power welding of 10mm or thicker steel plate. I did a Welding course once. Scary. Doing overhead welding not fun on 3/4" plate! No fancy automatic visors back then either.

[Russell W. Barnes]

<my two penn'orth>

ENW (Electricity North West) engineers will not even enter a substation without donning an arc-flash-proof impregnated boiler suit. I suspect other DNOs adopt the same policy. When you see what a faulty 11kV oil-filled breaker looks like when it 'goes off' it's little wonder.

There were some horror stories recently (last couple of years) about arc-flash and contact lenses, with the lens being fused to the retina when a flash was observed, leaving the wearer completely blind in the affected eye.

'Arc-Eye' feels like someone has chucked a bucket of hot sand in your face about eight hours after receiving a flash: very nasty indeed. When I was an apprentice electrician with British Steel about a hundred years ago, we were told the effects were ultimately harmless and would wear off. A lot of water has passed under the bridge since then, and now I'm not so sure. The worst I had was a headache from someone striking an arc behind a carelessly-placed screen. Many welders look suntanned around the collar / neck, with the UV from such arcs.

Measuring high voltage? Keep one hand in your pocket when possible. It's often not the shock that will kill, but the consequentials: falling off a ladder; banging your head, etc...

</my two penn'orth>