Electrical Question

Electrical impedance test

Hello, we have just moved into a new house and found a safety notice from British Gas saying that an earth impedance test was less than 100 ohms.After finding safety notice and talking to friends a couple of people have said that they got a similar warning after British Gas serviced their central heating -anyone heard of this happening ?
Please can anyone let me know what this means ?
Is it serious/dangerous ?
Is it likely to be an expensive job ?
- Many thanks.

3 Answers

Best Answer

Hi, Happy to answer this for you. The maximum zs value (impedance) on a particular circuit depends on the size of the over current device (fuse or mcb) that's supplying that circuit and the type and nature of the earthing system. (TNS, TNCS, TT etc etc) As an example, a 6 amp lighting circuit should be less than 2 ohms (ZS - Earth loop impedance) and a 32 amp ring main should be around the .80 ohm mark. If you have circuits that have a higher earth loop impedance reading, it means the circuit breakers take longer to operate during a fault condition. Meaning if somebody is being shocked, the circuit breaker will not turn itself off within the required time. For circuits less than 40 amps, the required tripping time is less than .4 of a second. Circuits over 40 amps is less than 5 seconds. If you don't have RCD protection on all of the circuits this makes the risk of injury or death more likely if somebody was shocked. Hope this answers your question. Alfie Hants Electrical Ltd

Answered 14th Feb 2016

Hants Electrical

Member since 24 Sep 2013

Well, a 6a lighting circuit, assuming the mcb is either a bs3871 (type 2) or bs60898 (type b) has a maximum Zs value (taking into account rule of thumb) of 5.82 ohms, the same type and bs number for a 32a mcb has its maximum set at 1.08ohms.

As for the maximum disconnection times, these at set at 0.4 seconds for all final circuits, and have nothing to do with the current that the mcb is rated to. Switchgear supplying distribution cables must operate within 5 seconds but that's only going to be found up to the main fuse and nowhere else. And they operate in the event of a fault, not just when somebody is on the receiving end of an electric shock. And while having an RCD does indeed improve safety, adequate earthing and bonding are far more important.

To answer the original question, British Gas are very good at marketing and sales. Get an electrician round to carry out a few checks, mainly just a Ze, and a PFC. It's possible that there is an earth fault in the distribution cable that needs repairing if a few houses are affected. An electrician who knows what he's doing, and knows how to test, would know in less than 10 minutes if there is anything amiss.


Answered 14th Feb 2016

Elmbridge Electrical

Member since 12 Nov 2012

You've been offered a lot of technical advice, some conflicting max values amongst it. Better to be safe than sorry, employ a registered competent electrical person with the relevant insurance and experience to conduct an Electrical installation condition report (EICR). This is a fact based report that should make it clear in laymans terms all you need to know.

Answered 24th Feb 2016

Amber Electrical Services NE

Member since 4 Apr 2013

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