Overcurrent Relay 50/51

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bejoyengineer

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Why is the overcurrent relay's - CT (Wired from the CT to the O/C relay), placed before a circuit breaker and why not it is connected after a breaker. this has been noticed with any bay -either the line bay or transformer bay.
 
It can be done either way as the 50/51 relays are non-directional. They are sometimes placed before the circuit breaker so that the space available after the circuit breaker can be used for a bus differential relay scheme.
 
Overcurrent relay

Overcurrent relay

So does that mean that if the 50/51 was a directional relay - then it should be placed before the breaker and if the answer is a yes, then why do we have to do so?
 
They're not, but if they were, the directionality would be based on where they have their polarization input from. That is why we use a 67 or a 32 relay. A 67 is an overcurrent relay with directional characteristics and a 32 is a power relay with directional characteristics.
 
richxtlc said:
It can be done either way as the 50/51 relays are non-directional. ---
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rich -
I must not be translating you right. What difference would it make which side of the CB the CTs were on - directional relay or not?

As you said, the CT polarity matters, but not whether the CT are line-side or load-side.

cf
 
Depending on the bus arrangement , maybe the CT's were placed ahead of the breaker to act as BUS OVERCURRENT CT'S . Just a thought . A one-line of the bus would be helpful .
 
I saw one breaker cubicle where the A and C phase CT's were on the line side of the breaker and the B phase CT was on the load side. This was how they fit in the gear.
 
Sorry for the confusion, when I talked about polarity I was talking about it in reference to directionality which is a function of a polarizing potential transformer or current transformer. The polarity of the current transformers is not important as long as they are all in the same direction. When using directional relays the polarity of the current transformers is extremely important. As far as whether the current transfromers are on the bus or feeder side is not important, they will function the same either way.
The system you describe may be a matter of physical space rather then relay protection. It would help if you could supply the wiring for the current transformers and the 50/51 relays.
 
It may sound like one, but it is not. The polarity marks on the cts can be placed in either direction as long as they are all in the same direction. If you reverse one ct you will get current in the neutral relay equivalent to 2x the current in the reverse phase.
 
bejoyengineer said:
Why is the overcurrent relay's - CT (Wired from the CT to the O/C relay), placed before a circuit breaker and why not it is connected after a breaker. this has been noticed with any bay -either the line bay or transformer bay.
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I does not matter. More the most suitable location is before the feeder ACB.
 
Cold Fusion said:
Why is before the "feeder ACB" a better location?

cf
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It includes the circuit breaker in the feeder zone of protection and the 87 which is usually located on the feeder side of the breaker includes the breaker in its zone of protection providing an overlap of protection for the circuit breaker. The overlapping zones of protection is the ideal way to provide complete protection for equipment.
 
richxtlc said:
It includes the circuit breaker in the feeder zone of protection and the 87 which is usually located on the feeder side of the breaker includes the breaker in its zone of protection providing an overlap of protection for the circuit breaker. The overlapping zones of protection is the ideal way to provide complete protection for equipment.
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You are correct in your description for a bus differential relay protection scheme but I believe the OP stated that this is just an overcurrent protection (50/51)scheme. In which case the conventional way is usually on the breaker line side. Unless their is no room to mount the CTs there then they locate them on the load side.

TT
 
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