Earth Fault

[Geoff1946]

The 250kohms is the insulation resistance. i.e. the quality of the insulation between the electrically live parts and earth. Earth being the exposed metalwork in your house, the boiler case, the sink, etc
The flaw in what the gas man has done is (we think) not separating the boiler from the incoming supply. Removing the fuse and switching off the spur isolates the live feed, but almost certainly not the neutral.
Hence his measurement will show any live or neutral to earth path in your house. There are often legitimate reasons for a low reading under theses circumstances, surge suppressed plugs or extension leads most common.
Provided you have RCD protection, ( a house your age must have) I would not worry, but I may try to find a better gas technician next time.

 

[Fleeting]

The switched fuse connection unit shown, unless faulty is double pole switched.

 

[Geoff1946]

The switched fuse connection unit shown, unless faulty is double pole switched.

Are you sure? I took it to be a single pole. If it's not I apologise.

 

[johnb2713]

Thanks for all the responses. Seems like the general consensus is that it doesn't sound an issue but should get an electrician to check.

For someone uneducated like myself, can someone explain (in layman terms preferably) what these readings actually mean please?

What your plumber is saying is that their is an electrical path from Live and from Neutral to earth, leakage of electricity. This test needs to be done properly for the readings to be meaningful. From what you have told us about this plumber, his electrical knowledge would seem to be somewhat lacking.

 

[Fleeting]

Are you sure? I took it to be a single pole. If it's not I apologise.

All switched fuse connection units must be double pole according to BS1363. That spur is quite new (Schneider) and it will be double pole.

 

[binky]

Thanks for all the responses. Seems like the general consensus is that it doesn't sound an issue but should get an electrician to check.

For someone uneducated like myself, can someone explain (in layman terms preferably) what these readings actually mean please?

it means that the circuit tested is not totally isolated from other appliances connected to the same cct. In such circumstances we tend to test at 250V Live and Neutral connected together, to the earth - stops potential damage to appliances from the tsting itself, as most appliances have 'smoothing circuits' to protect the control boards, all of which 'leak' to earth. So to confirm there is no fault on the circuit you actually need to disconnect everything on the circuit concerned, not just test at a random point.

 

[Fleeting]

Strictly speaking if that switch is off then it is isolated. We are assuming the plumber is wrong when in fact there maybe an issue.

 

[michael8554]

If the live wire in a cable accidently became shorted to the earth wire, then the ohms resistance between earth and live would be very low or even zero.
A component in the boiler that is going faulty might start to show lower than normal resistance between live and earth.
For example, this starts to happen with the heating elements in kettles, ovens, and immersion heaters.
Eventually they short out.
250,000 ohms will cause a leak to earth of only 1mA, one thousandth of an Amp.
The Residual Current Detector (RCD) breaker in your "fuse box" (Consumer Unit or CU) will only trip at a much higher value of 30mA, the value of "short to earth" considered to be becoming dangerous.
My "not an electrician" explanation.

 

[Almundoman]

Was he checking the cable going to the boiler from the spur or the cable feeding the spur from the Consumer Unit? I've never heard of a boiler engineer checking the wiring, surely that's the sparky's job? What test equipment did he use a generic uncalibrated multimeter? Also the fact he uses the term holms and not ohms really doesn't inspire any confidence in his ability to test any part of the electrical installation. Is anything else wired into the spur like a thermostat receiver or any kind of boiler controls or wiring? Is this an independent engineer or is he a company employee? Either way I would ask for an official report and then quiz him on his findings.

Thanks for all the replies. I've attached the details of the fault. It would be amazing if anybody could advise further please?

 

[Mad Toad Society]

The 250kohms is the insulation resistance. i.e. the quality of the insulation between the electrically live parts and earth. Earth being the exposed metalwork in your house, the boiler case, the sink, etc
The flaw in what the gas man has done is (we think) not separating the boiler from the incoming supply. Removing the fuse and switching off the spur isolates the live feed, but almost certainly not the neutral.
Hence his measurement will show any live or neutral to earth path in your house. There are often legitimate reasons for a low reading under theses circumstances, surge suppressed plugs or extension leads most common.
Provided you have RCD protection, ( a house your age must have) I would not worry, but I may try to find a better gas technician next time.

Thanks for the detailed response. I've managed to arrange for an electrician so hopefully he'll be able to sort it or at least explain it!

 

[Mad Toad Society]

What your plumber is saying is that their is an electrical path from Live and from Neutral to earth, leakage of electricity. This test needs to be done properly for the readings to be meaningful. From what you have told us about this plumber, his electrical knowledge would seem to be somewhat lacking.

Thanks for this. Yeah I've arranged for an electrician now

 

[Mad Toad Society]

it means that the circuit tested is not totally isolated from other appliances connected to the same cct. In such circumstances we tend to test at 250V Live and Neutral connected together, to the earth - stops potential damage to appliances from the tsting itself, as most appliances have 'smoothing circuits' to protect the control boards, all of which 'leak' to earth. So to confirm there is no fault on the circuit you actually need to disconnect everything on the circuit concerned, not just test at a random point.

Thanks for the explanation. So from reading this I also have a hot water tank which is connected to thr boiler. So perhaps he disconnected the boiler but didn't disconnect the hot water tank and hence the readings

 

[Mad Toad Society]

If the live wire in a cable accidently became shorted to the earth wire, then the ohms resistance between earth and live would be very low or even zero.
A component in the boiler that is going faulty might start to show lower than normal resistance between live and earth.
For example, this starts to happen with the heating elements in kettles, ovens, and immersion heaters.
Eventually they short out.
250,000 ohms will cause a leak to earth of only 1mA, one thousandth of an Amp.
The Residual Current Detector (RCD) breaker in your "fuse box" (Consumer Unit or CU) will only trip at a much higher value of 30mA, the value of "short to earth" considered to be becoming dangerous.
My "not an electrician" explanation.

Thanks for this. Afraid this this too technical for a simpleton like me. Sounds like if there is a leak, the rate it's currently leaking won't result in a short. I guess we need to establish why we are getting these readings and hope it doesn't get worse!

 

[Mad Toad Society]

Was he checking the cable going to the boiler from the spur or the cable feeding the spur from the Consumer Unit? I've never heard of a boiler engineer checking the wiring, surely that's the sparky's job? What test equipment did he use a generic uncalibrated multimeter? Also the fact he uses the term holms and not ohms really doesn't inspire any confidence in his ability to test any part of the electrical installation. Is anything else wired into the spur like a thermostat receiver or any kind of boiler controls or wiring? Is this an independent engineer or is he a company employee? Either way I would ask for an official report and then quiz him on his findings.

So I saw him switch off the boiler open it up and connected two prongs into the boiler and then there was a machine for the reading. He said the first thing on his checklist is to make sure the electricity is OK before he can carry out thr boiler inspection. I have a hot water tank and he didn't touch that. I'm beginning to think that might be the reason for the issues

 

[binky]

So I saw him switch off the boiler open it up and connected two prongs into the boiler and then there was a machine for the reading. He said the first thing on his checklist is to make sure the electricity is OK before he can carry out thr boiler inspection. I have a hot water tank and he didn't touch that. I'm beginning to think that might be the reason for the issues

So basically he tested the circuit board, hence the readings.

 

[Geoff1946]

I'm confused now.
He came to service the boiler, found a suspect fault with it, and went away suggesting an electrician looks at it?

 

[Mad Toad Society]

I'm confused now.
He came to service the boiler, found a suspect fault with it, and went away suggesting an electrician looks at it?

Yeah. So I get the boiler serviced each year. This time he said he needs to run through a checklist prior to carrying out the boiler checks. He said the first thing is an electrical test. And since that failed he can't carry out the boiler check

 

[Mad Toad Society]

Yeah. So I get the boiler serviced each year. This time he said he needs to run through a checklist prior to carrying out the boiler checks. He said the first thing is an electrical test. And since that failed he can't carry out the boiler check

And he recommended that I get an electrician to sort it out first

 

[Andy™]

And he recommended that I get an electrician to sort it out first

and if there isn't an issue with the electrics, will they be paying for it?

 

[Mad Toad Society]

and if there isn't an issue with the electrics, will they be paying for it?

So now the electrian is saying, from the basic checks there is nothing wrong. If the boiler engineer isolated the boiler and is getting those readings then there's something wrong with the electrics in the boiler itself and now we need a electrician that is qualified to work with boilers