ELI Reading above 200

[BaronGreenback]

I'm after some clarification please?

We have a TT system with an RCD, and as far as i can understand from the 17th regs (411.5) says that the maximum earth loop impedance on a TT system for 230V with a 30Ma RCD installed is 1667. It also says that anything above 200 would be unstable, with water and frost affecting its stability. Is this correct?

I ask, as today we had a repair guy turn up to fix a warranty fault on our tumble dryer.
As soon as we told him the fault, he said he didn't have the parts and never looked at the machine. Instead he plugged in an earth loop detector into a couple of sockets in the house (I wasn't around), and said as the results were above 200 he wouldn't be fixing the fault on the tumble dryer until we got a cert off a qualified engineer saying the electrics were safe.

Why he tested the home electrics and what's it got to do with a repair of a tumble dryer i have no idea, and i wish i was at home! As he had a pre-designed form to print off, I suspect this is common response. The form doesn't have an exact reading.

British Gas, replaced our meters a couple of months back with smart ones, and did various tests, so I would have thought that if there was a problem it would have shown up then.

I don't want to pay for a electrician only to find that there is no problem.

 

[JohnW2]

We have a TT system with an RCD, and as far as i can understand from the 17th regs (411.5) says that the maximum earth loop impedance on a TT system for 230V with a 30Ma RCD installed is 1667. It also says that anything above 200 would be unstable, with water and frost affecting its stability. Is this correct?

Yes, that's essentially correct. The note about 200Ω actually refers to the resistance of the TT earth electrode, but that will be only very slightly less than the total EFLI. One normally aims for an earth electrode resistance under 100Ω and, as you say, definitely under 200Ω.

This therefore really does need some attention - the problem is probably the earth electrode, but it could also be due to a poor connection somewhere. Where is your earth electrode - is it somewhere where the soil stays fairly moist?

Kind Regards, John

 

[BaronGreenback]

thanks for the quick response.

Yes - the earth electrode is at the front of the house - in a damp area, that's more hardcore/brick and sand than soil. But it's been totally frozen the last week - so i suspect this wouldn't help.

I'll have a look at the connections tomorrow - but i have no way of measuring if it makes a difference.

 

[flameport]

1667 is a theoretical maximum, based on the 30mA RCD and a maximum of 50 volts on exposed metalwork.

On real installations anything that high is totally unacceptable as soil conditions, weather etc. can have a significant effect on the impedance.
200 is suggested as a maximum, but in reality anything above 100 should be investigated - it's not difficult to get well below 100 in most situations.

 

[BaronGreenback]

would this have been something that would have been checked when the meters were changed?

 

[JohnW2]

thanks for the quick response. Yes - the earth electrode is at the front of the house - in a damp area, that's more hardcore/brick and sand than soil. But it's been totally frozen the last week - so i suspect this wouldn't help.

You're welcome, and fair enough. Although it's been seriously cold recently, if the rod is anything like a serious length much/most of it should be below the depth at which freezing of soil occurs in the UK, even with recent temperatures.

I'll have a look at the connections tomorrow - but i have no way of measuring if it makes a difference.

That's the problem. I fear that you're going to have to get in an electrician who has the necessary testing kit. As I said before, you don't want to leave it with an electrode resistance above 200Ω.

Kind Regards, John

 

[JohnW2]

would this have been something that would have been checked when the meters were changed?

I don't know what they're meant to do, but I've personally never seen it checked during a meter change.

Kind Regards, John

 

[Iggifer]

I've done 3 rods recently, all for temporary supplies, and the first rod has brought the reading to around 5Ω without any shenanigans like pouring water around it.

I wouldn't fancy driving a rod in this weather though!

Gravel/hardcore/sand is not the best material for an earth rod to be buried in. Definitely worth getting looked at

 

[Iggifer]

I don't know what they're meant to do, but I've personally never seen it checked during a meter change

I've only ever seen polarity checked, and I've even seen that checked with a plug in tester, rather than anywhere important. Completely pointless checking it at an accessory, hth does that prove the installations polarity is correct?

 

[BaronGreenback]

cheers

 

[JohnW2]

Completely pointless checking it at an accessory, hth does that prove the installations polarity is correct?

Strictly true - but I suppose it would require an extremely unlikely combination of errors for the polarity to be correct at an accessory if it were incorrect at the origin of the installation.

Kind Regards, John

 

[JohnW2]

cheers

You're welcome.

Kind Regards, John

 

[Iggifer]

Strictly true - but I suppose it would require an extremely unlikely combination of errors for the polarity to be correct at an accessory if it were incorrect at the origin of the installation.

Kind Regards, John

I don't know, more than once I've found RP at a socket yet the cable cores are actually what's RP and the socket termination is correct. You may (or may not, tbh) be surprised how many idiots there are. A guy that used to work with us 2nd fixed a whole kitchen with RP because he was used to the layouts for a certain brand of socket terminals and didn't bother to check the brand he was using properly.....

 

[JohnW2]

I don't know, more than once I've found RP at a socket yet the cable cores are actually what's RP and the socket termination is correct.

Well, yes, but that's just a single error. To have correct polarity as measured at a socket (without examining its connections) yet RP in the installation/cables would require two independent errors (both rare, despite the proliferation of idiots!).

Kind Regards, John

 

[ericmark]

The Martindale EZ150 costs around £35 and has three lights which can be off, red, or green giving 1.7 to 500 ohm not an exact reading, better than 100, 200 or 500 would not be good enough to fill in a report, but will give you some idea of what you have.