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#137216 - 06/12/03 09:50 PM FINAL SUBCIRCUIT DESIGN STRAGETIES
400volt Offline
Member
Registered: 05/19/03
Posts: 13
Loc: australia
What types of calculations are used to design final sub-circuits to meet the fault loop impedance requirement and the volt drop requirements in Great Britian, New Zealand, and or any other Country other than the Untited States.

Regards,
400volt.
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#137217 - 06/13/03 07:49 PM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
Trumpy Offline


Member
Registered: 07/05/02
Posts: 8560
Loc: SI,New Zealand
400V,
I can't say that I've ever installed a new sub-circuit, with Fault-Loop impedance solely in mind.
However, Voltage drop is always taken into account.
Regarding formulae, for calculating this,
I prefer to use this one:

Vd = lxIxVc
--------------
1000

Where l= Length of run(metres)
I= Circuit current(A)
Vc= mV drop per ampere-metre

There are also tables around for finding the voltage drop of a given length of run of a certain size of cable.
These are distributed by cable manufacturing companies.
Hope that this helps.

[This message has been edited by Trumpy (edited 06-13-2003).]
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#137218 - 06/14/03 03:10 AM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
pauluk Offline
Member
Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
BS7671 (IEE Regs.) basically specifies that the loop impedance must be low enough to open the fuse/MCB within a specified disconnect time. That time is 0.4 sec for general outlets, or 5 sec for lighting and fixed appliances.

Voltage drop here is permitted to be up to 4% from supply terminals to the end of the circuit.
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#137219 - 06/14/03 04:08 PM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
Dapo Offline
Member
Registered: 03/04/03
Posts: 52
Loc: Australia
Paul our standard AS/NZS 3000:2000 has the similar requirement to yours, I think ours is in fact based on yours.

Clause 1.7.4.3.4 Disconnection times
The maximum disconnection time for 230/240 V supply voltage shall not
exceed the following:
(a) 0.4 s for final subcircuits that supply—
(i) socket-outlets having rated currents not exceeding 63 A; or
(ii) hand-held Class I equipment; or
(iii) portable equipment intended for manual movement during use.
(b) 5 s for other circuits including submains and final subcircuits supplying
fixed or stationary equipment.
NOTE: Maximum disconnection times will vary for other voltages and
installation conditions.


Unfortunately our standard does not give the impedance values to meet the 5 s trip time with circuit breakers, is the UK standard anymore informative?

Edited to format the message



[This message has been edited by Dapo (edited 06-14-2003).]
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#137220 - 06/14/03 10:19 PM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
Trumpy Offline


Member
Registered: 07/05/02
Posts: 8560
Loc: SI,New Zealand
Dapo,
So pretty much, we are talking about Circuit-Breaker (Close) protection, here.
A porcelain re-wireable fuse would not be able to provide these levels of safety, hence they are banned over here, for any new work.
It would also rule out certain types(Q2 and R class)HRC fuses?.
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#137221 - 06/15/03 03:51 AM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
pauluk Offline
Member
Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Yes, the types of circuit defined in that AS/NZS are pretty much the same as for the U.K.

The actual limits on the loop impedance will depend upon the type of OCPD in use. As Trumpy said, the rewireable fuses take a lot more to "blow" than the equivalent rated cartridge fuse or MCB. For many years the IEE Regs. specified cable ratings for rewireable fuses with a footnote that ratings could be increased 33% for close protection with BS1361/BS88 cartridge fuses or MCBs.
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#137222 - 06/15/03 12:39 PM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
C-H Offline


Member
Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Quote:

Unfortunately our standard does not give the impedance values to meet the 5 s trip time with circuit breakers, is the UK standard anymore informative?


I don't think that it is possible to meet this with a type B or C circuit breaker, without meeting the 0.4 s criterion at the same time.
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#137223 - 06/15/03 05:08 PM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
David UK Offline
Member
Registered: 10/03/02
Posts: 134
Loc: Inverness, Scotland
C-H,
You are quite right regarding breaker loop values.
Table 41B2 of BS 7671 gives maximum loop values for Type B,C & D MCB's to BSEN 60898.
It states the values given comply with the 0.4s & 5s disconnection time.

400volt,
I have some excellent course notes illustrating cable selection procedures for compliance with BS 7671. They show calculations required for volt drop, shock protection, thermal constraints, application of correction factors (de-rating) etc.
I can't post them here as they are copyright of the training organisation & run to a few pages.
If you wish to e-mail me (contact PaulUK for my e-mail address), I will be pleased to scan the relevant section & send it to you.

[This message has been edited by David UK (edited 06-15-2003).]
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#137224 - 06/17/03 05:59 PM Re: FINAL SUBCIRCUIT DESIGN STRAGETIES
400volt Offline
Member
Registered: 05/19/03
Posts: 13
Loc: australia
Hi David from the UK that information sounds very interesting. Thanks. However, could you provide me with more details such as who produced them and how to contact them if that is possible.

Regards,
400volt
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