Ze Query

Hi Could anyone answer this query? We are looking to replace a cable that has gone down. The cable is fed from a section board (protected by a a square "D" CNAE34125 125amp) which in turn is fed directly off the HV transformer (11kv-433V) in the adjacent room (max 3m metres length of cable).

The supply cables to the section board are 500mm singles with approx 2X150mm earth.

The Ze is reading 0.17 at the incoming main switch. The problem is the manufacturers Zs reading for the 125amp MCCB is 0.18. The Zs reading at the D.B which is fed from this breaker is 0.44.

What Ze reading should we expect at the incoming supply? Under normal circumstances 0.17 would be great.

Any help on this would be appreciated, Cheers

500mm single that 1 big ass cable! what is the feeding?

Part of an Airport!!

Originally Posted by planetman

Hi Could anyone answer this query? We are looking to replace a cable that has gone down. The cable is fed from a section board (protected by a a square "D" CNAE34125 125amp) which in turn is fed directly off the HV transformer (11kv-433V) in the adjacent room (max 3m metres length of cable).

The supply cables to the section board are 500mm singles with approx 2X150mm earth.

The Ze is reading 0.17 at the incoming main switch. The problem is the manufacturers Zs reading for the 125amp MCCB is 0.18. The Zs reading at the D.B which is fed from this breaker is 0.44.

What Ze reading should we expect at the incoming supply? Under normal circumstances 0.17 would be great.

Any help on this would be appreciated, Cheers

What is the supply system type?

The ESQCR regulations 2002 suggest:

TN-S Maximum 0.80 Ohms

TN-C-S Maximum 0.35 Ohms

Edit, your transformer is in the adjacent room?

You're probably getting a bad reading on you EFLI tester due to the proximity of the transformer, check it's impedance on the rating plate and calculate Ze from that plus a negligible amount for the 3 metre cable run.


BS 7671 calls for an accuracy level of 0.05 ohm (5%), when measuring impedance. Most test instruments available on the market achieve this level, or better. However, when very low readings below 0.1 of an ohm exist, the test instrument can be outside of this requirement. This is because of the 2nd digit tolerance, which can vary between 4 and even 10 digits in some cases. For example; a meter reading of 0.05 ohm, measured with a meter whose tolerance is +/- 2% (+/-4 digits), could have a true value anywhere between 0.01 and 0.09 ohms. The reading would also not be consistent, if measured a number of times.

Thanks! Its TN-C-S, but whats causing me a headache is that Square "D" paper work says 0.18 max Zs, but i dont see how this is achievable without installing huge cabling!?!?!?!

YEs tansformer is in the adjacent room> ta

if i read the post correctly, it appears that you have an increase from0.17 to 0.44 on a 3meter run , this seems high. possibility of a high resistance connection? thinking about it, taking the reading at the squareD breaker, you have the resistance of the breaker itself adding to the result. this sounds like a more likely cause.

Originally Posted by telectrix

if i read the post correctly, it appears that you have an increase from0.17 to 0.44 on a 3meter run , this seems high. possibility of a high resistance connection?

I think the 3 metres is the transformer to main switch, he doesn't give a length for the distribution circuit.

Originally Posted by IQ Electrical

I think the 3 metres is the transformer to main switch, he doesn't give a length for the distribution circuit.

just edited my last post. could it be the resistance of the squareD breaker pushing up the Zs

Sorry for the confusion. 0.17 is read at the incomer to the Section board which is 3m away from the transformer. Th 0.44 reading is taken from the D.B being fed from the section Board which is 200m away and is fed on a 95mm 4-core. Cheers

Just to add more! the 0.17 reading has been taken on the live side of the incoming supply off the tranny. So with 500m singles and the run being less than 3m it is basically the reading off the tranny.

Just thinking about it< if a TN-C-S is allowed to be 0.35, how could we possibly expect a Zs reading as low as 0.18????? as the Square D paper work suggests (very confused )

beats me as well. if your Zeat origin were 0.35,a Zs of 0.18 would be impossible to achieve.