OT:- Trippin mains - Electricity gurus in da house?

Skin effect is totally unimportant at mains frequencies and domestic cuirrents, it's a real issue for me though when designing switched mode transformers and I have to use various techniques to get around it. Greg

I'm afraid your system is outside my experience, we have a system called protective multiple earthing (PME) which is the norm in newer builds and may be the same thing but I'm not certain. Our earth trips haven't monitored the earth current directly for a very long time, I've taken our 30 year old devices that didn't, there's no need as any imballance between phases and neutral must by definition be an earth leak, nor have our protective devices allowed you to hold them on for fairly obvious reasons.

Which sounds more like moisture to me, but who knows.

We have the option of either an RCD, MCB or both combined into an (expensive) single block called an RCBO but that's purely a practical matter, it lets you put an RCD on a single circuit on an existing board by replacing the MCB with an RCBO.

Which means that skin effect is insignificant unless a conductor is more than about 17mm in thickness or diameter, that's a very big cable (about 100x bigger than in a ring main) but does help explain why bus bars in large installations are wide and thin. Of course we're talking thousands of amps now, and once you get up to that sort of power levels you also go up to higher voltages so the currents are kept manageable. Greg

When I was at electronics college (Marine electronics, being trained to repair radar, radio, gyro platforms etc) one of our lecturers used to work on aircraft and used to tell various tales. One was of someone dropping a spanner across two bus bars used to provide current to the engine starter motors, he said the spanner electroplated everything within a 10-foot radius. Having dropped a spanner on top of the pinz 24-volt battery set and watched it dance a merry dance and lose chunks of metal I can believe it!

The story I heard about bus bars concerned telephone exchange batteries. Huge accumulator banks, and bus-bars just close enough to stick a finger in the gap.

Don't do it while you're wearing a ring.

Submarine batteries are also interesting.

Interestingly, in all the crap about the dangers of microwaves, I've never seen considerations of the lack of penetration into a realistically conductive human head, due to the skin effect. I know it depends on conductivity, but salt water is pretty conductive stuff.

Steve

I used to work on high power radio frequency gear, that doesn't electrocute you, it burns the flesh. We were told that bits of jewelry are to be avoided, particularly necklaces! We used to work on

Closest I got to injury from electric shock was when I was looking over a friend's shoulder and he got a belt from the mains and his hand, holding a screwdriver, shot past my head with about an inch clearance..

Because we were being trained to work on ships and communications gear, we were also taught about bombs including having to look at photos of bomb victims plastered all over rooms and suchlike.

ISTR issues about being gassed *and* electrocuted in the event of a sea water leak!

Austin, I have a 14cfm Clarke air compressor which has a 3HP single phase motor on it and at start up it draws about 29Amp's and once running is constantly drawing 12 Amp's. As a general rule of thumb motors at start will draw 3 times the running current. This is protected by a 32A MCB for the circuit for the compressor and the garage has a 30mA RCD for shock protection, I have never had a problem with tripping, hope this helps.

"I have a turbo diesel and I am not afraid to use it"

Rich

Very - the old man was a submariner and spent a short time on conventional boats before going nuclear. Someone shorted the batteries on that - luckily they were along side.....

I also worked with a ganger who had an "old sea dog" pock marked face - later I found out he been jumping a dumper when the battery exploded. Messey.

Richard

Nasty.

My busbar story comes from broadcast transmitters. Off duty transmitter engineer has a short dip in their home supply and jokes about the transmitter having a problem. Driving in later there is smoke coming form the generator house as the transmitters where running from them. Some how some one had managed to short out one of the incoming 11kV supplies. The bus bars where housed in a metal box the lid of which was neatly curved upwards between the fixing screws, the bus bars themselves were absent.

On or around Sat, 16 Dec 2006 11:56:05 +0000, beamendsltd enlightened us thusly:

Me grand-dad (no longer with us since about 1984) was in subs in the first world war.