Loop Impedance Test

[Edw1n]

We are doing replacement of 4 wire,415V rated overhead distribution pole. We are required to do the loop impedance test before energizing the line and was told by the utility that acceptable reading of 0.4 ohms (L-N)and report should it exceed this reading.

Can anyone tell me why is the maximum acceptable reading set at 0.4 ohms for the overhead line while for underground cable is 0.2 ohms

 

[Kiribanda]

Edw1n,

Could you please clarify whether you meant "Earth Fault Loop Impedance" specified by BS 7671 or some other?

Kiri

 

[Edw1n]

Hi Kiribanda
I would think that it would be the earth fault loop impedance otherwise is there any other type of loop impedance test other than the above.

 

[ausphil]

My only thought is that you are probably looking at different cross sectional areas for the overhead and underground lines. You generally need a higher cross section area for underground cables for the same current than you need for overhead lines, due to the heat dissipation, so you are probably looking at a lower resistance of the cable than the overhead line.

For example, for 95mm^2 Al cable, we would use a rating of around 250Amps, but to get 250Amps from an overhead line in aluminium, we could run about a 40mm^2 conductor.

The DC resistance ratios of these are about 2:1, so maybe that's why the figures are set with that proportion.

 

[Kiribanda]

Edw1n,

First of all the "Earth Fault Loop Impedance (Zs)" is a measurement that you have to take in an electrical installation wired with TNC or TNCS earthing. In other words Zs has to be measured through a socket outlet using a Earth fault Loop Impedance meter to make sure that the earth fault protection of the installation is provided by overcurrent devices installed. These are all as per BS 7671 and applicable for electrical installations in buildings.

Therefore I donot know how it can be measured on a 415 V overhead distribuitin systems using poles and what type of equipment to be used.

Therefore my reasonable guess is that your Utility may be specifying a loop impedance of 0.4 Ohms between any phase conductor and the neutral conductor to make sure your overhead conductor wiring is sound.

May be another learned member of this forum advise you.

Hope this helps.

Kiri

 

[Edw1n]

Kiribanda
If it is the sound wiring that the utility would like to ensure between any phase conductor and neutral conductor, my question is how they arrived at a maximum loop impedance requirement of 0.4 ohms for overhead and 0.2 ohms for underground.

 

[Kiribanda]

Edw1n,

I am sorry I cannot answer your question. I presume those values may have been decided by the utility by their own. Because usually Utilities have their own norms and specs.

Best way is to ask for a copy of their specifications/ standards relevant to your work.

Kiri

 

[hajji]

hi edw 1n

could you tell me how to do a earth loop impedance test in a domestic house.

any help much appericated

 

[ScottyUK]

If you have are UK-based and have to ask that question then under Part 'P' of the Building Regulations you shouldn't be doing the testing as you can't have passed the inspection course. But other than that it is much as Kiri stated: attach the loop impedance tester to the circuit and take the reading. The IEE's On-site Guide has a section testing too, starting at page 62 in the current edition.


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Sometimes I only open my mouth to swap feet...

 

[Marmite]

Generally low voltage overhead lines are in rural areas and tend to be long and spindly with small cross section conductor, say 35 or 70mm Cu. They would typically be fed from small pole mounted transformers, fused at 100-200A on the LV side. Bearing the topography in mind it is unlikely you can get anywhere near the .2Ohm mentioned for cables. Urban systems tend to be underground with fatter cables fed from large transformers and fused at 3 or 400A in the substation. The figures are the maximum values that the particular DNO permits. They originate from Engineering Recommendation P23-1 with is an Electricity Networks Association document. The DNO is required by statute to provide the information to anyone who has good reaon to require it eg electrical installation designers. They would use the information to design the downstream protection. The DNO's own fusing policy for its own network will also assume these maximum figures as the worst case. If you go outside the limits the fuse at the feeding pole mtd Tx or substation may not blow on a fault.
Regards
Marmite