House Wiring - 2

[JAS21]

I have started another thread as the last one (House Wioring) is a little old and has dealt with other issues.

The guys from SAFETY-T-CUT came yesterday. They installed an MEN link. I asked about cable sizes but they said a smaller size than supply is ok. They told me that, all this is via a 3rd party! of course. Also that for 5 years now it is the rule that all new builds must have an MEN link. I don’t think ‘rule’ means ‘law’.

I ask them to demonstrate that the ELB actually worked by testing at a socket. They did this in a not very precise way but to me it appeared to be sort of acceptable.

Pour water on the marble floor (it looks like marble anyway). Short between socket live and the water using a 3 watt light bulb (bulb in a packet presumably to stop them burning their fingers) and then measuring current with a clip on ammeter. They only got about 3mA in the living room. They said the floor was a good insulatotr

So went outside to Thai Kitchen and repeated. They got the ELB, which was set at 15mA to trip at 9mA. I should have checked the value with my fluke but didn’t.

Then they did the following with the MEN link disconnected. Shorted between the incoming live and the earth on the consumer unit, again using the 3watt bulb. They got 99mA and said that earth is not that good as should have got between 120mA and 200mA. AS I said beforethe earth rod is buried under the floor

This puzzles me a little as surely the bulb resistance is very high compared to the earth resistance or is this not the case.

I mentioned that I appeared to be short of earths, more mcbs than earth wires and this was neglecting the lights which I know do not have any earths. They said that probably earths had been commoned somewhere and that only the guy who wired the house would probably be able to explain what he did. Loft access is extremely risky..just a few RSJ’s and wires to hold up the ceiling. Guess the only way I will find out what is earthed and what is not is to isolate supple, disconnect earths at consumer unit and ‘ring out’ between every earth I find in the house and each earth at the consumer unit.

All comments/opinions very very gratefully received .

[Crossy]

The guys from SAFETY-T-CUT came yesterday. They installed an MEN link. Excellent!

I asked about cable sizes but they said a smaller size than supply is ok. So long as the cable is big enough for the rating of your incoming breaker 'tis OK.

Also that for 5 years now it is the rule that all new builds must have an MEN link. I don’t think ‘rule’ means ‘law’.

Pour water on the marble floor (it looks like marble anyway). Short between socket live and the water using a 3 watt light bulb (bulb in a packet presumably to stop them burning their fingers) and then measuring current with a clip on ammeter. They only got about 3mA in the living room. They said the floor was a good insulatotr

So went outside to Thai Kitchen and repeated. They got the ELB, which was set at 15mA to trip at 9mA. I should have checked the value with my fluke but didn’t. Better that it trips early

They got 99mA (equates to about 2000 ohms, too high) and said that earth is not that good as should have got between 120mA and 200mA (equates to 1100 to 1800 ohms, still really higher than I'd like to see). The use of a MEN link will assist to pull the trip should a fault occur but you really should sort the ground rod situation

This puzzles me a little as surely the bulb resistance is very high compared to the earth resistance or is this not the case. Tungsten lamps have a non-linear resistance, they are low resistance when cold, the resistance rises rapidly when the filament heats up, using a lamp is actually quite a smart idea as it limits the potential fault current should the trip not drop out.