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Topic Title: Advice on question
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Created On: 11 January 2013 06:53 PM
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 11 January 2013 06:53 PM
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Posts: 62
Joined: 02 November 2005

Hi all,

Today I was emailed the following question, I am not quite sure on how to answer this as I'm not sure it all looks correct but thought id ask your advise as my response will be picked at and commented on.
thanks in advance.


I wonder if you could offer me any advise on the following:

At NMH we have a Main Substation on a TT system. The Main Panel has six TP & N outgoing Fused Switches feeding a Sub Main Panel (SMP) in each student block A to F. 6 in total.

The protective devices for each SMP are 600A BS88 Type Gg fuses x 3.

The measured Zs at each panel is between 0.12 & 0.15 ohms.
The measured Ze at the origin (earth connected) is 0.11 ohms.

The maximum Zs for the above fuse, is I think around 0.03 ohms (I can't find out from the fuse manufacturer the exact Zs but for a TN-S system the value is 0.054 ohms, so I suspect I won't be far out).

Because the Zs figures far exceed the maximum allowable I can't issue a satisfactory certificate. I have asked the ECA for their advise on the matter but they won't commit.

Each SMP has 24 x SP 100A breakers (8 per phase). So the maximum demand of 800A has been reduced to 600A (aka the fuse size per phase). But to comply with the regs for max Zs I would have to fit 200A fuses, which is not practical.

Am I missing something or what! What would you advise?""
 11 January 2013 07:19 PM
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Joined: 23 March 2004

is it TT really ?

Effectively, you have a total failure of design - the distribution arrangements totally outstrip the supply capacity - essentially the TX isn't big enough for your distribution to work at maximum capacity and thus the impedance is higher than the design requires.

Time to start dropping fuse sizes I suspect - probably to reflect realistic loads.

How big is the supply TX - and how accurate do you believe the measured Zs and Ze values to be



Let the wind blow you, across a big floor.
 11 January 2013 08:09 PM
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Posts: 62
Joined: 02 November 2005

I am dubious on the TT, the readings seem a bit low, I will know more Monday as it will be discussed but I just thought I'd get some fresh ideas before going in. The installation is around 10 years old I think.

New here?

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