I touched a exposed live wire - Very Stupid!

[eveares]

I have never mentioned this before because of huge embarrassment of my own stupidity ; oh, and please go easy on me.

Anyway a couple of years ago when I had my extension done (you'll know if you've read my other posts) I had a rewire and a new CU as part of it.

Well before all the electrical and the new CU had been installed, I noticed a lose 2.5mm T&E cable tucked behind the board that the old CU was mounted to.

Anyway I knew all circuits were radial and that their was only 1 T&E cable from the CU per circuit. I then pulled this 2.5mm T&E cable out from behind, and because the conductors were bare/exposed and not terminated, I assumed the cable was dead.

I then had the bright and very foolish idea to grab hold of the exposed line and neutral conductors of this cable with my hand, what I incorrectly assumed was dead.

Oh boy did I get a shock and a fright from it.

Turned out that the cable came from the loft where it was wired into a live junction box what used to feed a FCU in the cupboard with the CU; but because it was no longer needed, it was tucked behind the CU with the conductors exposed!

Lucky I am still hear today.

Lesson - Use your brain and don't assume ever!

 

[PrenticeBoyofDerry]

Very stupid indeed and luckily your still with use.
I guess all those that work in the industry have had the odd tickle, whether through negligence of there own doing or of that of someone else.
But at the end of the day in almost all cases we are in the hands of our own destiny and a few extra seconds spent proving cables are dead is always worth the effort.

So who was responsible for leaving this hazard as it was?

 

[bernardgreen]

Even if the circuit has been proved dead still treat as possibly live.

Economy 7 and other time switched circuits can get you.

A dummer plumber was caught out when the thermostat operated. The even dummer electrician ( or plumber ) who installed the system had put the two "Isolate" switches ( one by boiler the other by the hot water tank ) in parallel. Both had to be off to isolate the circuit.


I too have had the occasional reminder.......

 

[JohnW2]

Even if the circuit has been proved dead still treat as possibly live.

Indeed so - but that is sometimes easier said than done - there is a limit to what one can easily do with insulated tools and/or (adequate) gloves.

I too have had the occasional reminder.......

I would imagine that virtually all of us have done, although I think I must be getting wiser with increasing age, since it's very many years since I last recall any such 'reminders'! In fact, in my case, most such reminders were not in relation to AC installations but, rather, in the distant days of my mis-spent youth (of dabbling with electronics and amateur radio equipment), related to high DC voltages - most commonly 250V - 500V but sometimes going over the threshold into the realms of HV (at voltages of 1500V-2000V DC) ... and the most common reason for those reminders was forgetting to discharge large capacitors after equipment had been powered down.

Kind Regards, John

 

[plugwash]

My most recent one was fixing a power supply in a BBC master, mains was disconnected and the primary capacitors did have drain resistors. Unfortunately one of the drain resistors had failed open circuit (which incidentally was why the power supply was misbehaving).

 

[JohnW2]

My most recent one was fixing a power supply in a BBC master, mains was disconnected and the primary capacitors did have drain resistors. Unfortunately one of the drain resistors had failed open circuit ....

I think one of the problems I experienced was due to the fact that, at high voltages, drain resistors can be very power hungry (hence generate a lot of heat) and difficult/expensive to source, so that 'hobbyists' tended to be rather 'mean' with their drain resistors. Consider a 100&#956;F capacitor charged to 1500V. To get decay to a fairly safe level (say, <50V) within, say, 4 seconds would require a resistor of about 11.76 k&#937;, which would dissipate about 191W during operation. That's an awful lot of wasted power and generated heat, and a suitable component would be difficult and expensive to source. Hobbyists therefore tended to use much larger value drain resistors (if any!), such that it could take 'minutes' for the voltage to decay to a safe level.

(which incidentally was why the power supply was misbehaving).

Interesting, and that rather surprises me, since I would have thought that the current drawn by the drain resistor would usually be very small compared with the normal current supplied by the PSU during operation.

Kind Regards, John

 

[plugwash]

It worked ok if started from a "capacitors discharged" situation but it wouldn't reliablly come back up after a power cycle.

I suspect the asymetry of the situation (as with many power supplies of that era there were two capacitors in series to allow for a 120V option, each of which had a seperate drain resistor) didn't help.

 

[bernardgreen]

Some large electrolytes would generate a voltage across their terminals a few minutes after they had been discharged and then left unconnected. The voltage was sometimes enough to give a good zap to fingers touching the terminals.
The mechanism was a small amount of charge remained and the dielectric coating on the plates, created by the charging, dissolved back into the electrolyte. Less dielectric means the capacity is reduced thus increasing the voltage due to the charge remaining..

Voltage = charge / capacity.

 

[JohnW2]

Some large electrolytes would generate a voltage across their terminals a few minutes after they had been discharged and then left unconnected. The voltage was sometimes enough to give a good zap to fingers touching the terminals. ... The mechanism was a small amount of charge remained and the dielectric coating on the plates, created by the charging, dissolved back into the electrolyte.

Yes, I remember (and have suffered from!) that phenomenon! Similar can happen with seemingly 'fully discharged' batteries - but the voltages involved aren't then usually enough to create any 'feelable' issues!

Kind Regards, John

 

[JohnW2]

It worked ok if started from a "capacitors discharged" situation but it wouldn't reliablly come back up after a power cycle.

Interesting.

I suspect the asymetry of the situation (as with many power supplies of that era there were two capacitors in series to allow for a 120V option, each of which had a seperate drain resistor) didn't help.

As you say, that may have had something to do with it. In the sort of scenarios I was talking about, we often had two or more capacitors in series - simply because high value very high voltage electrolytics were difficult to find and/or beyond our pockets. In that situation, of course, the resistors across each capacitor were at least as important for trying to ensure the intended voltage sharing (so that none of the capacitors blew up!) as their role as 'drain' resistors!

Kind Regards, John

 

[eveares]

I honestly don't know what got to me and why I did not prove dead, but sure was not one of my brightest moments.

 

[ban-all-sheds]

In fact, in my case, most such reminders were not in relation to AC installations but, rather, in the distant days of my mis-spent youth (of dabbling with electronics and amateur radio equipment), related to high DC voltages - most commonly 250V - 500V but sometimes going over the threshold into the realms of HV (at voltages of 1500V-2000V DC) ... and the most common reason for those reminders was forgetting to discharge large capacitors after equipment had been powered down.

A friend of mine broke a finger as a result of one of his reminders - he jerked his hand away so forcefully that he banged it really hard against the inside of the TV cabinet.

 

[JohnW2]

A friend of mine broke a finger as a result of one of his reminders - he jerked his hand away so forcefully that he banged it really hard against the inside of the TV cabinet.

I recall a particularly potentially lethal TV. It had the telling model number of "TV1" (Bush, Phillips? - I can't really remember) and came in a floor standing wooden cabinet with something like a 10"-12" screen.

Nearly all CRT TVs derived the 'EHT' supply for the CRT (typically 10-15kV for B&W, more more colour) from a large winding on the line output transformer ('LOPT'), taking advantage of the rapid change in primary current which occurred during the 'flyback' period at the end of each line. As such, the current-supplying capabilities were very limited - such that, although nasty, EHT shocks were rarely fatal.

In contrast, perhaps because they hadn't yet thought of the 'LOPT' approach, the "TV1" had a full-blown conventional mains-powered 15kV power supply - with very appreciable current-supplying capacity and therefore potentially lethal.

Kind Regards, John