Series MV Relay and Fuse Coordination

[rockman7892]

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?

 

[waross]

The fuse protects the system, not the transformer. The fuse has to withstand the energization surge. Generally the fuse clears transformer internal faults. The fuse should not clear on secondary faults except in the case where the primary fuse is the only protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

If they're in series with nothing else tapped off in between then I'd try to set the relay such that the relay beats the fuse to the extent possible. Those fuses aren't inexpensive and when they blow that's the only information they provide. The relay can provide lots of information about why the breaker tripped.

 

[LionelHutz]

Well, the relay is just the detection device so it isn't actually in series with the fuses. It is used to trip a MV device in series with the fuses which is capable of being tripped via an external signal. So, you first need to know which device is being tripped, and are if there are any limitations on when it should be tripped. When you should trip a switch or contactor is different than when you should trip a breaker. Generally, a switch or breaker is only be tripped on overload allowing the fuse to provide short circuit protection while a breaker can be tripped whenever you want.

 

[rockman7892]

Warros - I'm not exactly sure what you mean by "the fuse protects the system, not the transformer" It was always my understanding that the fuse located at the transformer (MV fused switch) was used to protect the transformer from internal and through faults while the upstream relay was used to protect the system (feeder cable) as well as provide transformer protection.

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.

 

[ScottyUK]

Lionel,

" Generally, a switch or breaker contactor is only be tripped on overload... "

Edit your post if you have time, I'll delete this post.

 

[LionelHutz]

Way past editing time, but yes it's a contactor.

 

[unclebob]

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.

 

[rockman7892]

unclebob

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?