grading of overcurrent relay
grading of overcurrent relay
(OP)
When grading the overcurrent relay of an earthing transformer(zig zag) with other overcurrent relays downstream,what is the criterion to be followed?is it to be made more sensitive than others i.e it should trip before other overcurrent relays dowstream or it should be treated as ordinary overcurrent relay that will trip after the downstream relays(graded over them)?.Please help if anyone can.What if instead of OVercurrent relay it is earth fault relay?Thanks in advance.






RE: grading of overcurrent relay
According to our expirience not needed any grading of overcurrent protection of zigzag with other overcurrent relays.
Actually phase current of zigzag very very small.
You need provide grading according to the EF relays of others. And of course must be grading of EF relay of zigzag with otheres.
Hope that help.
Good Luck.
Slava
RE: grading of overcurrent relay
Regards
Marmite
RE: grading of overcurrent relay
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Cory Anderson
http://startco.ca/
RE: grading of overcurrent relay
Actually phase current of zigzag very very small.
So as far as overcurrent relay grading is considered,all other relays should be graded over o/c relay of zigzag transformer.Is that what you mean?please clarify.
RE: grading of overcurrent relay
No needed any grading of zigzag overcurrent relay with other relay and other overcurrent relays with zigzag overcurrent.
Please pay attention on the Marmite's and Cory's post.
Reason of zigzag is virtual ( sorry for term) ground reference. That menas, if zigzag will trip fast, the other earth fault protection will not operated.
Very important point what is your application:
zigzag is directly grounded, via resistor, via petersen coil?
It's a point of zigzag protection, becouse this ground fault current is a maximum "load" current of zigzag.
Your set of overcurrent protection must be higher then this current and time of ground fault protection is longer than
ground fault time of others.
Please don't take in account overcurrent protection of zigzag in grading of overcurrent protection of downstream and upstream feeders.
And of course, don't forgot about:
"In my experience earthing transformers are rated for a short time current for 30 secs, and are generously proportioned to withstand the associated heating and magnetic forces. NER's are typically rated for 10 secs."
Best Regards.
Slava