Series MV Relay and Fuse Coordination
Series MV Relay and Fuse Coordination
(OP)
Is there a rule of thumb or best standard practice for coordinating a MV relay and fuse on the primary of a transformer. Most applications I come across have an upstream relay ahead of the transformer feeder along with a MV fused switch usually located at the primary of the transformer.
Although these devices are in series and technically don't have to be "coordinated" but I was wondering what the rule of thumb was for coordinating these devices especially in the instantaneous region of the relay.
For the relay the Instantaneous is usually above the transformers secondary max let through so that id does not pick up for faults on secondary but never really was able to determine how its set in relation to the fuse curve. Should the relay INST setting totally clear the fuse curve in the INST region if possible or should there be a certain clearance margin down at 0.01s where the fuse goes into current limiting?
Although these devices are in series and technically don't have to be "coordinated" but I was wondering what the rule of thumb was for coordinating these devices especially in the instantaneous region of the relay.
For the relay the Instantaneous is usually above the transformers secondary max let through so that id does not pick up for faults on secondary but never really was able to determine how its set in relation to the fuse curve. Should the relay INST setting totally clear the fuse curve in the INST region if possible or should there be a certain clearance margin down at 0.01s where the fuse goes into current limiting?






RE: Series MV Relay and Fuse Coordination
Bill
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"Why not the best?"
Jimmy Carter
RE: Series MV Relay and Fuse Coordination
RE: Series MV Relay and Fuse Coordination
RE: Series MV Relay and Fuse Coordination
davidbeach - You make a very good point about fuse replacement and the relay being able to provide more fault diagnostics. In this case I guessing you would want to set the Instantaneous setting relay above the secondary maximum fault current but below the fuse curve as much as possible. Obviously if the fault is large it may pickup the relay in INST region as well as cause fuse to blow in current limiting range simultaneously. Is there a good rule of them as to where the relay INST setting should cross fuse curve (as long as its above secondary fault current)or do you simply just try to set it below the fuse curve as much as possible? Do you ever use INST delay on relay on primary of transformer?
LionelHutz - I'm assuming that you meant only a switch or "contactor" is tripped on overload allowing fuse to provide short circuit protection.
I know some people subscribe to the philosophy of coordinating these devices so that a fault can be isolated to its location depending on which device trips but I don't know that will tell you much depending on the nature of the fault.
RE: Series MV Relay and Fuse Coordination
" Generally, a switch or
breakercontactor is only be tripped on overload... "Edit your post if you have time, I'll delete this post.
RE: Series MV Relay and Fuse Coordination
RE: Series MV Relay and Fuse Coordination
Depending if you have a current limiter or expulsion fuse, you may want to turn the INST off or max with a limiter fuse. As with an expulsion one, I would adjust it to clear the inrush to try to keep the AF energy to a minimum.
RE: Series MV Relay and Fuse Coordination
Is there a reason why you would want to turn the INST off with a current limiting fuse but set it lower with an expulsion fuse?
Is it to reduce Arc Flash and equipment damage when an expulsion fuse wont clear as fast?