Dangerous DIY

[bernardgreen]

The thread about an external socket from a light switch reminded me of a DIY bodge.

The elderly house owner complained that the new fuse box that she had been forced to accept kept turning all the power off.

Someone in the past had taken Permanent Live from a switch using a single core to a new switch ( indoor ) that controlled an outdoor lamp and on in a single to the lamp. The "neutral" for the lamp was a ground ( can't recall what, rod or metal water pipe ) adjacent to the lamp.

 

[ericmark]

And this caused a danger because? It likely resulted in false meter reading and there is an issue if the conductors are not insulated to required standard but with a proper earth pit around the stake so it can't be touched it's not really a danger just non compliant.

Today it would trip the RCD but years ago it was not really a problem.

I remember the post office used earth a lot around the 1960's OK only 50 volt but same idea. And on the Falklands earth return was used for telephones even into the early 1990's. The problem was wind caused static build up so telephones would not work on windy days.

The Post Office the earth return was something to do with party lines and as they disappeared so did the use of earth rods. They were normally steel not coated and had a bare conductor quite thin around 1mm stranded.

Where the danger arose was when an electricians (so called) used the telephone earth as the main house earth. Best of it the earth loop impedance was within limits for a TT and I wonder how many physically check an earth wire back to the rod.

I also had a fluctuating reading with the earth loop impedance on one job and found loose earth bolts. Again if I had not pressed the button twice and got two different readings I could have missed it.

The modern ELI meter uses around 8mA to test low enough to not trip the RCD. 612.2.1 tells us we should use at least 200 mA and at this there is a chance of locating a poor connection. Old ELI testers would get quite hot so I would assume they did take a fair current but when we were talking about the Plug-in socket testers and I looked up the spec and was surprised how little current was drawn. I then looked up ELI testers the idea was to point out how much more current they would draw. But although double that of a plug-in tester it was still well under 15mA which could trip a 30mA RCD.

 

[deadshort]

Perhaps her husband was a car mechanic ? Negative earth !

Regards,

DS

 

[ericmark]

I had quite a few fires with earth return wagons and drivers fitting CB's. They would take a wire to centre of the two batteries and the most negative and use this to feed the CB. The CB aerial was bonded to the metalwork of the cab. But with earth return the chassis is floating and not connected to positive or negative however copper and carbon dust would often cause the starter motor to end up earthing the engine and it was pure chance if positive or negative. If the former then it would burn out the braid of the aerial wire setting the roof lining on fire.

One would hope a mechanic would be very aware of earth problems and would avoid those problems.

 

[bernardgreen]

And this caused a danger because?

A dry earth would leave the rod ( if it was a rod ) Live and in close proximity to true ground. Also voltage gradients across the ground could be hazardous.

I remember the post office used earth a lot around the 1960's OK only 50 volt but same idea.

That was a signalling earth, not a power supply earth. For party line operation the earth impedance could be 400 ohms and still be useable. Watering the earth was a standard procedure when pressing the "Call Exchange" button did not call the exchange.

It is true that telephones can use earth return but cross talk will occur if two lines are in use at the same time and share the same route through ground.

 

[JohnW2]

And this caused a danger because?

A dry earth would leave the rod ( if it was a rod ) Live and in close proximity to true ground. Also voltage gradients across the ground could be hazardous.

Indeed. The ground would not even have to be particularly dry. If it were a 100W lamp and the resistance of the earth rod/spike/whatever were 100Ω (which is far from impossible, even for a proper, non-dry, TT rod), the top of the rod (and connection to it) would be about 37V above (true) earth. If the resistance of the earth rose above 100Ω, the voltage relative to earth could become hazardous.

Kind Regards, John