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Determining Max Zs For Time Current Curve

Former Community Member over 4 years ago

How do you determine the max Zs when the manufacturer does not list or specify the maximum permitted earth fault loop impedance? TN system.

https://library.industrialsolutions.abb.com/publibrary/checkout/GES-6119D?TNR=Time%20Current%20Curves%7CGES-6119D%7CPDF&filename=GES6119.pdf

0 Chris Pearson over 4 years ago

Consult BS 7671? ?
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0 Former Community Member over 4 years ago

Chris Pearson:

Consult BS 7671? ?

Does BS7671 take these curves into account?
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0 mapj1 over 4 years ago

Note that the figures in the BS are far too pessimistic. Real maker's data is often an order of magnitude quicker - so the apparent let-through energy is not as high as BS7671 suggests, and reduced CPC T and E is OK after all. Whew..

Mike
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0 Former Community Member over 4 years ago

In this example, do I just take the maximum clearing time at face value for determining disconnection time?
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0 David Stone over 4 years ago

If you do not wish to use the BS7671 figure, you can calculate the disconnection time from the manufacturer's worst-case energy let through. However, this is a deviation from BS7671 and needs to be fully documented and noted on the EIC. It is probably not worthwhile unless you are really stuck. The alternative is additional supplementary bonding to reduce R2, or choose a larger cable.
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0 Former Community Member over 4 years ago

davezawadi (David Stone):

If you do not wish to use the BS7671 figure, you can calculate the disconnection time from the manufacturer's worst-case energy let through. However, this is a deviation from BS7671 and needs to be fully documented and noted on the EIC. It is probably not worthwhile unless you are really stuck. The alternative is additional supplementary bonding to reduce R2, or choose a larger cable.

Is there anyway I can just use the above time current curve?
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0 mapj1 over 4 years ago

yes. Decide how fast you need it to break, safety of life at 240V, 110V whatever, will give you 0.4 seconds, 0.2 seconds or sometimes 0.1 seconds. Run your finger along that timeline, and read off the current multiple on the 'always breaks' side of the shaded region.

This is the fault current you need, to always be sure to open the circuit in that time.

Then use ohms law to see what resistance gets that fault current from your supply voltage (assume supply could be as low as 80% of nominal)

That's it.

M.
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0 AJJewsbury over 4 years ago

Don't forget Cmin these days. 411.4.4's requirement is that Zs x Ia ≤ Uo x Cmin.

Where Ia is the current causing automatic operation of the device in the required time, and for a conventional DNO supply Cmin is 0.95.

- Andy.
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0 Former Community Member over 4 years ago

Thanks. Do I use the maximum clearing time or go somewhere in the middle between the two boundaries of the time current curve?
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0 mapj1 over 4 years ago

Use the longest time that may occur and assume you need a fault current that exceeds the 'all trip' condition when considering how low Zs needs to be in respect of ADS for safety of life, or it may not prompt trip when you need it to.

Use the shortest trip time and least tripping current that may fire the device when designing a hierarchy of breakers trying to ensure that downstream devices trip first and this one does not.

Hint, with breakers you often cannot achieve this nested discrimination as easily as with fuses.

Mike
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Determining Max Zs For Time Current Curve

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