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Setting for 51N for 400 V Switchgear

Setting for 51N for 400 V Switchgear

Setting for 51N for 400 V Switchgear

(OP)
My question was related to Setting for 51N for 400 V Switchgear.
- 3 x CT of REF 542+ get from of 3 phase.
- this ground system is Solidly Ground.

How many percent of In(nominal)should be?
our setting is 0.05In and 0.7s time but many time tripped for unwanted tripping.
Is it very low ?
Can I adjust it to 0.1 In?
and What standard i can refer?

Thanks for answers

Young Engineer

RE: Setting for 51N for 400 V Switchgear

Hi.
It's very low.
Put about 0.12-0.13InIn, time is OK.
10% from CT size is minimum setting.
Best Regards.
Slava

RE: Setting for 51N for 400 V Switchgear

0.05xIn is low.

What is "unwanted" tripping? Main tripping when there is a fault on a branch/feeder breaker or tripping on just unbalanced current in the neural without any fault?

If set up as Neutral Over Current, your 51N needs to be higher than your anticipated maximum unbalance.  

If set up as earth fault protection, you  need to coordinate with largest branch breaker without a earth fault protection.

It appears that you in IEC world and your local codes would govern the earth fault protection.

If any guide, in the IEEE world, NEC permits up to 1200A and up to 1.0 second of time delay on 480V systems (similar to what a 400V system would be) for the earth fault setting. Although, I believe because of UL, most trip units would not allow more than 0.5 sec of time delay.

Although not specifically in the code, this relates to a maximum (and very common) service size of 4000A. So that equates to 30%, max.

You need to check your local codes.
 

Rafiq Bulsara
http://www.srengineersct.com

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