What could be the cause? 2400v feeder fuse

[EEatwork]

We have 2400V feeder line feeding 2 motor-driven pumps (also 2.4KV). The line side fuse popped on one phase and we replaced and everything worked; just a day or two later on the same feeder another fuse at different phase popped. We meged the line side feeder to see if we got a ground, it checked out good; also the load side line to the motor, and it also checked out good. now we suspect there might be something in the motor, so that's our next step to check, but my questions here is:

how come the fuses for each motor (we have 2 motors) are good but the feeder fuse popped first? I think it should be the other way round, cause the feeder line fuse should be rated higher than that of the motor right?

what do you think the cause for this is?

Thanks

 

[bob]

What was the voltage that you used to megger the feeder? What are the fuse sizes?

 

[dbuckley]

I know it sounds daft, but they could just be old fuses that have done their time of heating up...?

If seen PoCo fuses go before customer fuses on many occasions, and the suspicion is always old fuses.

 

[zog]

We have 2400V feeder line feeding 2 motor-driven pumps (also 2.4KV). The line side fuse popped on one phase and we replaced and everything worked; just a day or two later on the same feeder another fuse at different phase popped. We meged the line side feeder to see if we got a ground, it checked out good; also the load side line to the motor, and it also checked out good. now we suspect there might be something in the motor, so that's our next step to check, but my questions here is:

how come the fuses for each motor (we have 2 motors) are good but the feeder fuse popped first? I think it should be the other way round, cause the feeder line fuse should be rated higher than that of the motor right?

what do you think the cause for this is?

Thanks

Too much current for too long a duration

Seriously, we need more info. What are the fuse types and ratings? They may be miscoordinated. What was your test voltage for the motor megger test? What was the result? ("Good" dosent mean much) Any othe rloads involved? Take any current readings after you replaced the first fuse and restarted the motors?

 

[EEatwork]

Too much current for too long a duration

Seriously, we need more info. What are the fuse types and ratings? They may be miscoordinated. What was your test voltage for the motor megger test? What was the result? ("Good" dosent mean much) Any othe rloads involved? Take any current readings after you replaced the first fuse and restarted the motors?

the fuse that burned out is this old style that is actually fully submerged in an oil tank rated 200A. don't know the fuse for each motor but should have smaller ratings.

 

[Besoeker]

We have 2400V feeder line feeding 2 motor-driven pumps (also 2.4KV). The line side fuse popped on one phase and we replaced and everything worked; just a day or two later on the same feeder another fuse at different phase popped. We meged the line side feeder to see if we got a ground, it checked out good; also the load side line to the motor, and it also checked out good. now we suspect there might be something in the motor, so that's our next step to check, but my questions here is:

how come the fuses for each motor (we have 2 motors) are good but the feeder fuse popped first? I think it should be the other way round, cause the feeder line fuse should be rated higher than that of the motor right?

what do you think the cause for this is?

Thanks

If there is no ground fault, the fault current would have to have gone through the fuses in the other phases in some proportion which may have weakened them.
In that sort of situation we would have generally replaced all three fuses.

Doesn't explain the initial failure though.

 

[brian john]

Can you check the start stop counts. We had this once and the motor was starting and stopping excessively, resulting in fuse failure.

 

[jim dungar]

If protective devices do not operate in their intended sequence, it is almost always a case of mis-coordination.

Each protective device family has its own Time Current Curve (TCC). Coordination, between devices, exists only if the TCC's do not overlap. Simply comparing the FLA of protective devices is not sufficient to make a judgment on coordination.

 

[zog]

If protective devices do not operate in their intended sequence, it is almost always a case of mis-coordination.

Each protective device family has its own Time Current Curve (TCC). Coordination, between devices, exists only if the TCC's do not overlap. Simply comparing the FLA of protective devices is not sufficient to make a judgment on coordination.

Exactly! Thats why I was asking for types.

 

[jim dungar]

The line side fuse popped on one phase and we replaced and everything worked; just a day or two later on the same feeder another fuse at different phase popped.

The first occurrence 'weakened' the fuse(s) that did not open. I believe the recommended practice for MV fuses is to replace all 3 phases at the same time.

 

[EEatwork]

gathered some more info and findings related to this case

* when the 350HP motor is running, it makes each phase current on the 2.4KV feeder uneven; 55, 25, 46 amps on each phase; What could be the cause for this and could this be contributing anything to the fuse blown? when the other 200HP motor runs, currents on each phase are close to each other tho.

* 5kv megger was used to meg the line side and load side; so the measurments should be credible

* we run those motors about 10-15 times/mo.

* feeder fuse is the oil cutout type; rated 200A

 

[zog]

* 5kv megger was used to meg the line side and load side; so the measurments should be credible

What were the results of the megger test?

What are the fuse type? Every fuse has a type and a TCC, just knowing the amp rating dosent help. Also need the types and ratings of the other fuses. Liek Jim said,m sounds like a coordination issue but we dont have enough info to help yet.

 

[jim dungar]

* feeder fuse is the oil cutout type; rated 200A

What are the other fuses?
Under oil fuses can have different trip curves. Is this an overload fuse or one for short circuit protection?

One phase at 55A and one at 25A is not a good thing.

 

[EEatwork]

What were the results?

lineside we have phase-to-phase between 8 - 10Gohms, phase to ground is around 4Gohms

loadside we have phase-to-phase around 100Mohm and phase to ground between 25-40Mohm.

 

[EEatwork]

One phase at 55A and one at 25A is not a good thing.

so what could be the cause for this?

 

[EEatwork]

What are the other fuses?
Under oil fuses can have different trip curves. Is this an overload fuse or one for short circuit protection?

One phase at 55A and one at 25A is not a good thing.

2.4KV feeder fuse is GE's Oil Cutout Fuse Link (OCFL) 9F57CAA. still trying to find the specs on this one. but this is the model type.

 

[Besoeker]

so what could be the cause for this?

A problem in the 350HP motor could be.
Probably not a fault to ground since you have good IR readings.
Possibly an open circuit winding?

 

[jcole]

Are there any reduced voltage starters (wye-delta, reactor, autotransformer,etc.) or electronic starters (softs start or VFD)? I assume there probaly is. The problem could be here especially if it is an old reduce voltage starter .

 

[DavisIMI]

Kinda of the subject though, I once worked at a very large auto plant, (no names), and I had an electrician that was making some changes in an LPP, ( lighting power panel), and anyway he grounded his screwdriver out against the pnl as he was tightening up the breaker,(277V), instead of this blowing the fuse in the buss plug feeding the pnl, it tripped the main GFI breaker that fed that one building, it shut that whole building down. Luckily it was after hours and no harm done except for some pride broken. I thought well the plant E-engineers will be glad this happened because their had to be a coordination problem, no, they said that was the way it should be.???? I cant believe that the main breaker that feeds the whole plant, that building, which was huge, robots and all, instead of blowing the fuse on the buss plug that feeds the pnl thats 25' away. What say you?

 

[jim dungar]

...the plant E-engineers will be glad this happened because their had to be a coordination problem, no, they said that was the way it should be.???? I cant believe that the main breaker that feeds the whole plant, that building, which was huge, robots and all, instead of blowing the fuse on the buss plug that feeds the pnl thats 25' away. What say you?

It is very hard to coordinate a single ground fault device with any breaker greater than about 30A. Above 100A there is almost no GF coordination possible. This is one reason that hospitals must have at least 2 levels of GF, if they have any at all.