Line-Loop Testing

[Bradley6969]

Hello all,

Just got a quick question for you all.

Recently noticed our engineers have been advised to start testing line-loops at every piece of equipment. They are recording the highest and noting this within the comments section on each circuit schedule of test results.

Other than systems conforming to TT and to verify the overcurrent device will operate for any short circuit faults. It seems they are also coding the results based on what results they get.

For example - B63 60898 distribution circuit has a maximum Zs = 0.72 Ohms. If they get higher than this on the line-loop they will also code as C2.

How would I cross reference the results they get?

I was always told if the cable size is adequately sized to the overcurrent device this would be sufficient for any short circuits.

Thanks all.

 

[littlespark]

I use the following guide, which I downloaded from the internet. The information is from the 18th edition of regulations.

Circuit breakers and fuses also have a “type” as well as current rating… which is related to their trip time characteristics.

Your example of a 63A 60898 circuit breaker is for a Type 1, not B… which is 0.58 ohms

Comparing the measured readings to this guide has been part of an EICR for years. They should have always been doing this… not just started recently.

 

[newfutile]

I think these are old values not taking into account cmin .

 

[Bradley6969]

Sorry I may have not been clear on my original post.

I’m on about the line loop testing between line conductors (line+neutral).

We have only recently started testing this and recording the results.

Obviously I know how to compare the results for the maximum earth fault loop impedance.

Is it any different for line loop testing?

 

[westward10]

I would suggest you can use the same Tables what difference would it make?

 

[Bradley6969]

I didn’t know if there was another calculation?

 

[littlespark]

You mentioned Zs at the start which is earth loop impedance… so you can see where the confusion is.
There’s also the r1 or the r1+r2 to measure but afaik, there’s no entry in bs7671 model forms for anything else like that.

(Not including the IR and rcd times as they are different tests altogether. )

 

[PS Electrics]

I was always told if the cable size is adequately sized to the overcurrent device this would be sufficient for any short circuits.

Do you mean if the cable size satisfies Zs for a certain protective device then it will also satisfy for a short circuit?

This would be correct because the cpc is in most cases smaller than the neutral conductor (or the same size) and so if the Earth Loop Impedance value will result in a satisfactory disconnect time then a short circuit will also by definition.

Which is why #5 is correct in that if your tested values are lower than the Zs values in the table the circuit complies.

Also a lot of comments on this on the IET forum are saying

"There is no shock hazard from an L-N fault, so the disconnection time is not based on the need to prevent electric shock,"

Which backs up #7 and why it isn't on the model forms and there is no requirement to test it.

What is the purpose of doing the test? Disconnection on short circuit is more about not damaging the cable or equipment and causing fires. Testing the impedance of L-N can be useful to determine circuit efficiency but to make any meaningful deductions you would need to know the cable length to compare.