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Essential Safety Tips for Working with Electrical Circuits

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Re: Essential Safety Tips for Working with Electrical Circuits

Postby IvanSC » Wed Sep 10, 2008 8:36 am

Hugh Robjohns wrote:You're right. The important message is what can happen, not the detail of voltages.

But who on earth managed to mis-wire the mains supplies so utterly incompetently, and are they still in business, or banged up in prison for attempted manslaughter?

Hugh
Have`nt managed to contact any of them yet, but apparently it was a smallish private "do" so who knows WHO was allowed to do the wiring.
I agree with you 100% - hopefully the survivors are now carrying their OWN safety kit, like I have for years!

The only thing I can think of here is that somehow they wound up being on 2 different phases at the same time, but surely an RDF would have seen this immediately and killed the power? The idea that anyone would hook up a temporary 3 phase supply without automatically fitting some sort of safety devices (particularly with the weeather we have been having recently) makes me shudder.
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Re: Essential Safety Tips for Working with Electrical Circuits

Postby Folderol » Tue Dec 02, 2008 12:25 am

Might have been a faulty neutral. Get this happen quite a lot on 3 phase machinery. As long as the phases remain in balance nobody notices, but as soon as someone puts a significant load from one phase to the (faulty) neutral all bets are off.
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Re: Essential Safety Tips for Working with Electrical Circuits

Postby ef37a » Tue Mar 09, 2010 10:17 am

Not really a safety problem as such, well, for the lashup perhaps...

Always fit an on off switch in the supply line(s)even for the most crude of rats nests so you can bang it off at the first sign of smoke.

Also worth having a couple of LEDs setup with a few K in the positive leg to tack in to show various operations, e.g. small 5V relays cannot be seen or heard to operate.

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Miswired three phase

Postby damoore » Sun Aug 08, 2010 2:21 pm

(Yikes.

The mains tester I carry just tells me that there is an actual earth (not waving its leg in the air) and that active and neutral are the correct way round.

Its just got three bulbs on it. There's a placard with expected "correct" display (two ambers) and the various faults. None of them say "you are across two phases of a three phase".

I wonder what that would display. Especially if, as well as being across the phases, it was miswired as well - anybody who can get it that wrong can't even be trusted to get the earth right.

Maybe it indicates this condition by emitting smoke?

Were they running through surge protectors? Be useful to know if they were and they failed to give protection.
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Re: Miswired three phase

Postby James Perrett » Sun Aug 08, 2010 7:09 pm

damoore wrote: None of them say "you are across two phases of a three phase".

Remember that standard practice is totally different in your part of the world and (as I understand it) things like tumble driers are commonly wired across two phases of a three phase supply in the US.

If you don't have a good knowledge of the standard wiring practices of the country that you are in then leave all mains wiring to someone who does. There are massive differences between the US and UK.

James.
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Re: Essential Safety Tips for Working with Electrical Circuits

Postby ROLO46 » Tue Feb 22, 2011 6:49 pm

Howabout the Newbury Races horse deaths recently?
What happened there? :angel:
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Re: Essential Safety Tips for Working with Electrical Circuits

Postby intense » Wed Apr 13, 2011 11:58 pm

ROLO46 wrote:Howabout the Newbury Races horse deaths recently?
What happened there? :angel:

There doesn't appear to have been much further news after the incident, perhaps because of the possible big bucks involved and concerns about liability. From what was reported it sounded like the insulation had broken on a disused underground mains cable that hadn't been isolated, and current to earth created a large enough potential difference at ground level to kill the two horses. All the stuff in the reports about the other horses being saved because they were wearing aluminium shoes couldn't have been right though - the danger zone was probably very localised and they missed it, or got away sufficiently quickly.

Evidently the fault current was insufficient to blow a fuse or trip a circuit breaker, which is strange considering that the cable should have been armoured and the metal armour strands connected to earth at both ends and, probably, to neutral at the supply end. But then again, the cable shouldn't have been live in the first place so it might not have been intentionally connected via a protection device.

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Re: Essential Safety Tips for Working with Electrical Circuits

Postby dmills » Thu Apr 14, 2011 3:40 am

Horses and cattle are notoriously prone to electrocution from ground voltage differences simply because the 4 legs are a reasonable distance apart.

There is actually considerable guidance in the 'special locations' section of the wiring regs dealing with stables and similar locations for exactly this reason.

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Re: Essential Safety Tips for Working with Electrical Circuits

Postby Folderol » Sat Jun 18, 2011 7:49 pm

More for protecting what you're working on than for yourself...

Make a pair of needle probes.

To do this solder a conventional sewing needle to a test lead in place of the normal probe (it is surprisingly easy to solder to the eye of a new needle). Try and make this joint as small and smooth as possible.

Then use the insulation from a thin wire to make a sleeve that goes over the needle itself right up to the joint and only leaving 2-3mm of the point exposed. Hold this in place with heatshrink tubing going over the joint and about 10mm either side. Put another piece of heatshrink on top of this going about 15mm either side, so you have a small firm 'probe' that's reasonably well insulated - not suitable for high voltage work!

Your new probes are ideal for testing chips and surface mount stuff as they are small enough for you to actually see what you're hitting and far less likely to accidentally bridge across two legs. Also, the very sharp points will easily punch through dirt and oxides, and even most PCB varnishes.

With care, you can also use them to spike through the insulation of ribbon cables etc. when you have possible faulty connectors.

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After you've stabbed yourself with them a few times, you'll learn to respect them :D
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Re: Essential Safety Tips for Working with Electrical Circuits

Postby ef37a » Sat Jun 18, 2011 8:05 pm

Folderol wrote:More for protecting what you're working on than for yourself...

Make a pair of needle probes.

To do this solder a conventional sewing needle to a test lead in place of the normal probe (it is surprisingly easy to solder to the eye of a new needle). Try and make this joint as small and smooth as possible.

Then use the insulation from a thin wire to make a sleeve that goes over the needle itself right up to the joint and only leaving 2-3mm of the point exposed. Hold this in place with heatshrink tubing going over the joint and about 10mm either side. Put another piece of heatshrink on top of this going about 15mm either side, so you have a small firm 'probe' that's reasonably well insulated - not suitable for high voltage work!

Your new probes are ideal for testing chips and surface mount stuff as they are small enough for you to actually see what you're hitting and far less likely to accidentally bridge across two legs. Also, the very sharp points will easily punch through dirt and oxides, and even most PCB varnishes.

With care, you can also use them to spike through the insulation of ribbon cables etc. when you have possible faulty connectors.

P.S.
After you've stabbed yourself with them a few times, you'll learn to respect them :D

Lot of work Will?
I just used to get something like this: http://www.maplin.co.uk/moulded-4mm-test-probe-leads-5665

and tickle the ends up on a grinding wheel (now I have to do it laboriously with a file!).

Also a good idea to makeup a jack plug and jack socket to 4mm conns, saves a lot of bad language when interfacing with amps, AI's and such.

Dave.
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Re: Essential Safety Tips for Working with Electrical Circuits

Postby Folderol » Sat Jun 18, 2011 10:24 pm

Not at all Dave, I've attached a piccy of the probes I made quite a few years ago. I used the leads from a pair crappy probes - just cut them off. Must have taken me all of 15 minutes.

The points have never needed sharpening, and will poke through just about anything, unlike ordinary brass ones. They also don't have the ridiculously long exposed metal that standard probes have.

I have to watch the other engineers as they are inclined to attempt to 'borrow' them, and our apprentice is very pleased with himself have recently made a pair of his own, only he stuck to the 2mm exposed length (having recently experienced the death of a chip).

P.S.
These probes are used every day!

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By the_real_folderol at 2011-06-18
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Re: Essential Safety Tips for Working with Electrical Circuits

Postby Folderol » Tue Aug 16, 2011 6:27 pm

Just want to say...
If in doubt - don't!
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