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Topic Title: DNO Supplies
Topic Summary: DNO supply Ze
Created On: 14 April 2010 01:01 PM
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 14 April 2010 01:01 PM
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michaelross

Posts: 2
Joined: 05 November 2004

Hi, wondering if anyone can offer some advice

I have an installation where the main switchboard is approximately 150m away from the 8 distributions boards it is supplying. The supply authority are giving us a 250amp TP&N supply and have suggested they will supply an earth connection but this will be maximum 0.35ohms (as stated in the regs) - running cable calcs based on this value means massive CPC's are being detailed between the main switchboard and the distribution boards and we are having problems with earth loop impedance as to get 250/315amps fuses at the source to trip the Zs is required to be around 0.16ohms.

Can anyone advise on how/what the power authorities are likely to install at the source with regards to fuse sizes, realistic Ze value, disconnection times expected/required for their equipment etc

Thanks

M

 14 April 2010 01:20 PM
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OMS

Posts: 18919
Joined: 23 March 2004

I have an installation where the main switchboard is approximately 150m away from the 8 distributions boards it is supplying. The supply authority are giving us a 250amp TP&N supply and have suggested they will supply an earth connection but this will be maximum 0.35ohms (as stated in the regs)


If you know the supply can deliver 250A within a reasonable voltage tolerance you can realistically predict Ze Particularly on a TN-C-S.

we are having problems with earth loop impedance as to get 250/315amps fuses at the source to trip the Zs is required to be around 0.16ohms.


Aplogies if I misundertsood this but as your total supply capacity is only 250A anyway where did a requirement for 315A devices come from

Equally if you are feeding 8 Dist Panels at the load ends from this 250A supply then clearly there should be no requirement for each device to be anywhere near 250A if the downstream installation is reasonably designed

Can anyone advise on how/what the power authorities are likely to install at the source with regards to fuse sizes, realistic Ze value, disconnection times expected/required for their equipment etc


Probably a 400A cut out assembly with 315A BS 88 fuses on board. I wouldn't worry about disconnection times on thier kit - they have substantially a much more pragmatic approach to cable temperature rise under fault on thier network. Again, for your 250A fuses in the main switch, then why bother - an on load isolator is all you need.

Regards

OMS

-------------------------
Failure is always an option
 14 April 2010 01:33 PM
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michaelross

Posts: 2
Joined: 05 November 2004

Thanks OMS,

The reason I had previously said 250/315Amps was due to the fact I was unsure what the supply authority would fuse at.

The 8 DB's loads will be significantly below the 250Amp supply and the distribution is practically designed based on the loads, the only issue I have is with the supply cables to these DB's and the fact that large additional CPC's are being detailed in the calcs due in part to the high Ze (if using the 0.35ohms as stated by the DNO) at the source.

You mention in your post that the Ze can be realistically predicted - how is this done/is there a rule of thumb?

Thanks for your help

M
 14 April 2010 02:11 PM
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OMS

Posts: 18919
Joined: 23 March 2004

The supply voltage can vary from 253 down to 216 volts - ie 37 volts. You have a supply capacity of 250 A

From Ohms law we know that:

R = V/I = 37/250 = 0.15 Ohms

Regards

OMS

-------------------------
Failure is always an option
IET » Energy » DNO Supplies

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