Test Procedures

[BS3036]

I have been reading through Section 7 of Guidance Note 3, which roughly corresponds to Section 612 in BS 7671. I was trying to make some sense of the tests to see how logical it was.

What I don't seem to be able to find, is any case of a measurement of continuity of a neutral in a radial circuit.

The resistance of all three conductors is measured in the case of a ring final circuit, but in the case of a radial the neutral never seems to get measured. The only possible place relates to volt drop, but this is a fairly vaguely specified requirement.

It seems strange that the regulations would be suggesting that the way to know that a neutral in say a lighting circuit is checked by 'seeing if the lights work'. As a badly made high resistance connection could be missed, and potentially cause a fire.

Even in the case of a ring final, the neutral resistance is not actually recorded.

So, what have I missed?

 

[ricicle]

You haven't missed anything - I have always found it strange that there is no requirement to test the Neutral on radial circuits for continuity, especially considering the amount of Neutral burn offs/loose Neutrals we encounter.
A test of this nature may go some way to reducing Neutral problems !

 

[oharaf]

I sometimes link out neutral and earth on radials and lighting just to check

 

[ban-all-sheds]

Nothing to stop you checking RN if you want to....

 

[BS3036]

Nothing to stop you checking RN if you want to....

Of course not. It seems a pretty basic requirement. I just question how many thousands of committee man-hours went into writing a book of regulations which leaves out such a basic item, whilst mandating the rest of it.

 

[ban-all-sheds]

IIRC, don't NICEIC EIC schedules have a space to record RN?

 

[NotHimAgain]

Of course not. It seems a pretty basic requirement. I just question how many thousands of committee man-hours went into writing a book of regulations which leaves out such a basic item, whilst mandating the rest of it.

Is it a basic requirement - by that do you mean that is easy to do, or do you mean that it is essential for safety?

What exactly are you hoping to confirm from your "measurement of continuity"?

What does measurement of continuity mean - a circuit is continuous or it is not. Do you actually mean measurement of resistance?

What characteristic would you like the neutral to have: i.e are you satisfied if it is simply continuous; or would you like it to be both continuous and capable of carrying its full load current indefinitely; and also the prospective fault currents without suffering damage until the fault is interrupted.

The people on the 'wiring regulations committees' usually try to avoid asking for pointless tests to be carried out that prove little or nothing. If they felt that testing 1) was practicable in all installations, and 2) was the best way to prove of strength and capability of a circuit - I think they might just have said so. If a test for strength and capability were to be call for it would not be satisfied by an instrument that you can pick up with one hand (unless of course you want to use my favourite method - a spanner thrown on to the bus bars .

Note that for installations covered by the Health and Safety at Work Act - putting electrical equipment into service that does not have sufficient strength and capability for its intended use is an offence. Regulation 5 of the Electricity of Work Regulations is absolute. A simple continuity test does not prove strength and capability.

So what do you do?

 

[Spark123]

I'm thinking in a radial, as the neutral conductor is either shorter, larger CSA or the same size as the other conductors measuring RN doesn't really tell you much.
The only real benefit would be should there be a L & E reversal which was missed in the initial verification.

 

[Taylortwocities]

Is it a basic requirement - by that do you mean that is easy to do, or do you mean that it is essential for safety?

What exactly are you hoping to confirm from your "measurement of continuity"?

Lets see.
You do your R1+R2 test and get a value of (lets say) 0.50ohms

If you were then to do Rn+R2 and found a value of 1.00 ohms you would want to go find where that high resistance connection was!

If you didnt then your live Zs value would be OK but eventually (under load) that loose neutral connection would fizz away and fail.

 

[NotHimAgain]

Well first the R1 + R2 test is not a requirement of BS 7671 - but assuming you like doing it - what does it actually tell you?

Now you could go on, as you describe, and measure Rn + R2 and it is just possible that your results might identify a problem. However that would very much depend on the accuracy of your test method.

Just out of interest do you know what the allowable accuracy is for the test methods detailed in BS 61557-4:2007 - Resistance of earth connection and equipotential bonding - that is the standard that BS 7671 refers to.

Note that I have not quoted a standard for measuring the resistance of conductors generally (such as the neutral or a ring conductors) because there isn't one. Such tests are not required or recognized in the BS 61557 series.

However, the tests in BS 61557-4 can be carried out using the sort of instrument most electricians have. So what is the permitted accuracy of the testing method? These tests could be used as part of a system of verification - but only if their limitations are well understood.

I have pointed out that in installations covered by the EWR it is an offence to install equipment that does not have sufficient strength and capability - either because the equipment is not adequate or because it has not been installed properly - do you think the tests you describe discharge, either fully or partly, your duty under the EWR?

 

[Adam_151]

IIRC, don't NICEIC EIC schedules have a space to record RN?

No, but they do have a box for rn (along with r1 and r2), which I think is a good idea, also have proper box for 5x rcd time

 

[ericmark]

It is something I found when first doing inspection and testing. My meter had both prospective short circuit current and earth loop impedance and a single lead. I had wrongly assumed the ELI would be between Line and Earth and PFC between line a neutral and carefully noted both readings.

When I go down to entering the readings into a spread sheet I though it would be a good idea to compare the two readings so volts/Amps = ohms so that's what I did and every reading lined up when using 240 volts in spread sheet.

So I made a new test lead to check the PFC. On moving firms I ended up with a Robin all singing dancing unit and this did check line neutral for PSC so seems different manufactures have different ideas!

So even if it was included in the test there is a likely hood that in many cases it would not be done with electricians thinking they had tested it but the meter testing only the Earth Line PFC and not Neutral Line PFC.

On the forms we are only required to measure PFC at origin. But can't see how we can work out how unless we measure PFC the volt drop can ever be worked out?

Allowing for 6% on supply and 5% local the PFC needs to be more than 290A for a 32A supply but since not measured not sure how anyone knows if the volt drop is within limits?

 

[Adam_151]

Eric, a lot of the testers about require you to connect both neutral and earth leads to neutral to measure PSCC (as opposed to PEFC)

Mine has two settings L-E impedance and L-N impedance, and on both settings a display button toggles between impedance in ohms, and calaculted fault current

 

[ericmark]

I now realise that but in early days in late 20th century I have looked at the kettle type connector to 13A plug lead on the Siemens tested and assumed the correct cables were selected within the meter.

In fact on the latter Robin model they were.

So it would be very easy for an electrician who first used one that did select the correct cable to test to not realise some do not.

Of course when using flying leads we all know which to connect but with the provided easy connect leads mistakes can happen.

However since IET/BS have not included the reading on the sample paper work I think it's unlikely that many electricians test anyway! Working in house and having problems with volt drop we tested not to comply with regulations but to ensure the machines ran well. The shrink wrap machine was very voltage dependent and like a lot of factories things got moved around. So we temp labelled "Do not use shrink wrap machines in this socket" and as time permitted up graded cable.

But this is hardly likely to happen in domestic!

 

[bernardgreen]

For what is worth the old chap who from whom I learnt a lot in the 1960's did his testing with a 6 volt motorcycle battery and some low ohm resistors.

Each circuit was tested with DC at the rated current of the fuse that would be feeding it using the battery and the necessary resistors in series till the ampmeter showed the required current . The earth if installed was also tested at that current. He then measured the voltage drop along the circuit being tested. Two separate meters, one for volts, one for amps. He had a maximum acceptable voltage drop on the circuit which I cannot recall but it had to be less than 6 volts as that was the maximum drive voltage he had available.

Insulation was checked using the mains supply and first an amp-meter and then down to milli-amp-meter.

By today's standards crude in terms of equipment used but it did verify that the installation was safe. One thing his method of testing did do was to find "weak" sections of wiring where the impedance value was OK but where a short length of the circuit was of smaller cross section than was necessary to carry fault currents safely.

But then that was yesteryear.