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Topic Title: .4 or 5secs earth loop
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Created On: 25 January 2017 11:34 AM
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 25 January 2017 11:34 AM
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leeadams

Posts: 10
Joined: 02 September 2013

If applicable due to distribution circuit is the max ze value higher than that if it wasn't a distribution circuit ?
If so from what table are those values listed ?
 25 January 2017 12:00 PM
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mapj1

Posts: 9716
Joined: 22 July 2004

For MCBs it makes little odds, unless you have very high inrush types D, W or Z, as it is not possible to reliably trip a B or C type in 5 seconds or less using the thermal part - it is either a near instant magnetic trip or no thermal trip at all for some minutes.

For fuses, running a thumb down the maker's data sheet reveals the current for a 5 second trip time, and then divide 230 by that, or 80% of 230 if you like a little margin, as is now fashionable.

Of course if you have an RCD upfront, then earth faults are handled by that and for L-N faults, there is no need to clear that fast, as we are not worrying about safety of life, only prevention of fire.

edited for clarity to change 'slow blow' MCBs to 'high inrush' MCBs

-------------------------
regards Mike


Edited: 25 January 2017 at 12:47 PM by mapj1
 25 January 2017 12:07 PM
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AJJewsbury

Posts: 16117
Joined: 13 August 2003

Distribution circuits in TN systems have a maximum permitted disconnection time of 5s (reg 411.3.2.3) - likewise for final circuits rated above 32A. Final circuits rated 32A or below need 0.4s (reg 411.3.2.2 & table 41.1). For TT systems it's 1s for distribution circuits and finals >32A (reg 411.3.2.4) and 0.2s for smaller final circuits (table 41.1 again).

For many overcurrent protective devices (e.g. fuses and type D circuit breakers) 5s disconnection times can be achieved with significantly higher earth loop impedance values (Zs (not Ze)) than for 0.4s disconnection. For type B & C MCBs the values are usually the same (as you'd need to trigger the magnetic element in both cases - the thermal element wouldn't trigger fast enough without much higher currents that would trip the magnetic element anyway).

- Andy.
 25 January 2017 12:09 PM
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AJJewsbury

Posts: 16117
Joined: 13 August 2003

For actual Zs values see tables 41.2 to 41.4.
- Andy.
 25 January 2017 03:46 PM
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leeadams

Posts: 10
Joined: 02 September 2013

yes I meant Zs apologies well spotted.
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