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Topic Title: Ground Fault Protection and Discrimination
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Created On: 12 November 2013 01:43 PM
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 12 November 2013 01:43 PM
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fzn10

Posts: 16
Joined: 22 September 2013

Hi all,

I am looking through an existing installation and have noted that ground fault protection has been provided within the Main Incoming Switchboard only, the ACB is Masterpact NW40 H2 4000A Micrologic 6.0.

No ground fault protection has been provided within other switchboards downstream of this main switchboard.

Given that no ground fault protection has been provided downstream of this switchboard, am I correct in the saying that there will likley be ground fault descrimination issues with this?

For example, the ground fault protection for the 4000A Breaker is at 1000A(Max Setting), some downstream breakers are rated at 1250A, which would entail that that 1000A Ground Fault Breaker will not ground fault discriminate against the standard 1250A MCCB.

Many thanks
 12 November 2013 03:25 PM
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MickeyB

Posts: 181
Joined: 18 January 2003

Time delay on 6.0 is enough to discriminate for most devices although some larger MCCB's will cross for the short time delay. Although the instantaneous element should pick up first for higher faults. You can get a small zone of overlap. ..... for a very narrow range of faults.
 12 November 2013 05:38 PM
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OMS

Posts: 19463
Joined: 23 March 2004

With a micrologic 6.0 electronic tripping unit, you should be able to set short and long time settings that can discriminate with just about anything downstream with virtually no overlapping of curves.

Basically, you know how the 1250A units will operate for the range of earth fault currents they are exposed to, you can then set the micrologic to suit

Regards

OMS

-------------------------
Failure is always an option
 13 November 2013 06:10 AM
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fzn10

Posts: 16
Joined: 22 September 2013

Thanks for the responses.

The above responeses are true for overcurrent., Phase-Phase and Phase-Neutral Short Circuits, however I am referring to ground Fault discrimination(Ig and Tg Setting within the ACB.) which has a different curve all together and can only be set to 1000A Maximum. Is there a way to post pictures on this forum? Think it will be beneficial to show the discrimination curves.
 18 November 2013 04:48 AM
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fzn10

Posts: 16
Joined: 22 September 2013

Any one know that answer the query? Thanks
 19 November 2013 01:23 PM
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AJJewsbury

Posts: 11284
Joined: 13 August 2003

The above responeses are true for overcurrent., Phase-Phase and Phase-Neutral Short Circuits, however I am referring to ground Fault discrimination(Ig and Tg Setting within the ACB.) which has a different curve all together and can only be set to 1000A Maximum.

Won't the downstream overcurrent devices still operate on a L-PE fault in much the same way as a L-N fault? (Presumably this is a TN rather than TT installation?)

As for pictures, you can post links to pictures (using the button or "img" tags), but you need to find somewhere else to host the image (some here use photobucket).

- Andy.
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