High Zs on 800A MCCB

[tallpault]

High looking for a bit of advice might be picking this up incorrect.
A main supply panel board with an 800a and 400a mccb feeding a factory an office block respectively.
Tested Ze 0.07 ohms
Production Building Zs 0.04 ohms
Office Block Zs 0.07 ohms
Length of runs from panel board to both buildings are 130m and are both bonded down to the steelwork.
The max permitted Zs reading for an 800a mccb is 0.028.

How are you meant to achieve readings as low as 0.028 with a Ze of 0.07 ohms or as i say am i reading this wrong.
Any help would be gratefully received.

 

[johnboy6083]

with a supply of 800A, you woul;d usually have a transformer nearby, and as such your Ze should be very low. I would ring the DNO. It does sound high for a supply of that size.

 

[Knobhead]

That’s way off scale. How far away is the transformer?

 

[Engineer54]

High looking for a bit of advice might be picking this up incorrect.
A main supply panel board with an 800a and 400a mccb feeding a factory an office block respectively.
Tested Ze 0.07 ohms
Production Building Zs 0.04 ohms
Office Block Zs 0.07 ohms
Length of runs from panel board to both buildings are 130m and are both bonded down to the steelwork.
The max permitted Zs reading for an 800a mccb is 0.028.

How are you meant to achieve readings as low as 0.028 with a Ze of 0.07 ohms or as i say am i reading this wrong.
Any help would be gratefully received.

Just out of interest, where did you get your max Zs requirement for the 800 MCCB??

 

[malcolmsanford]

Talpult also if your using a bog standard MFT then you starting to go down the scale to a point where your not going to get a decent results because the lack of resolution at this level.

There is a great sticky on here by IQ on how you can combat these low value levels using a wander lead, which might give you a more accurate Ze, Zs readings.

also as E54 was that 0.028 the manufacturers value

 

[johnboy6083]

i went online, and found that Zs for a square d, 800A MCCB is very similar to what the OP stated.

 

[tallpault]

The transformer is 5m away form the main panel board.The supplier provided a 3 phase and neutral supply and we were instructed to link out the neutral and earth terminals within the panel board by the supplier which was done .Obviously giving us a tncs system
There is an acb main switch rated at 1000 amp feeding the the 800 and 400 amp mccbs.
The switchgear is abb and we contacted the panel board manufacturer who sent us the max zs readings.
thanks for you guys input.

 

[Knobhead]

It will be TN-S unless the DNO have linked the neutral at the transformer. In which case they wouldn’t have asked you to link N-E, they would have provided the earth.

 

[netblindpaul]

tallpault,
When you say supplier, do you mean electricity supplier, DNO, or panel board supplier?

 

[Engineer54]

As Tony has quite correctly pointed out, your N-E connection is the transformers Neutral being earthed. This is quite a normal practice on a dedicated DNO supplied transformer supply. So if you are experiencing high Ze values you can add extra earth rods to the transformers grounding ring. You really should be aiming for less than 10 ohms, obviously, the lower you can get your Ra values the better will be your final Ze value.

As a matter of interest, what size is the TX, and what sort of transformer protection do you have on your main switchboard??