There is a 100A 3 phase disconnect that keeps blowing a 100A fuse on phase A. The 3 phases only carry 50-60 Amps each. The fuse holds for about 3 days then the customer calls and says the fuse has blown again. There are no signs of excessive heat in the disconnect or the 100A panel that it feeds. The 100A panel it feeds has a main breaker which never trips, its allways the phase A 100A fuse that blows. Could this be an AC condensor or a walk in cooler condensor causing the problem? Any ideas would be appreciated.
Fuses keep blowing
- Thread starter mrhanky
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Fuses keep blowing
You didn?t describe the type of facility. Is this a restaurant? Laundromat? Store?
I would start by asking questions, in the hope of discovering an event that is common to all trips. I?d look for something that they did just before the trip. Look for something that they don?t do many times a day, but rather do once every 2-4 days. You might discover, for example, that they have an employee go over the floors with a buffer twice a week, discover that the receptacle used for that buffer is on Phase A, and then discover that the buffer has a ground fault. What you seek to learn is, ?What was happening just before the trip??
Next, I?d have a close look at the 100 panel. Is it labeled? Do you believe the labels? Can you tell what equipment is on Phase A? If so, I?d look into the equipment that does not operate constantly, but rather is operated once or twice a week.
Not true. It may be true when you take your measurements. But the behavior that you describe tells me that at some point in time, Phase A is drawing more than 100 amps. It may be caused by something that turns on infrequently, and that draws the excessive current when it does turn on. It may be caused by a weak spot in the insulation that comes infrequently into contact with a grounded point.
You didn?t describe the type of facility. Is this a restaurant? Laundromat? Store?
I would start by asking questions, in the hope of discovering an event that is common to all trips. I?d look for something that they did just before the trip. Look for something that they don?t do many times a day, but rather do once every 2-4 days. You might discover, for example, that they have an employee go over the floors with a buffer twice a week, discover that the receptacle used for that buffer is on Phase A, and then discover that the buffer has a ground fault. What you seek to learn is, ?What was happening just before the trip??
Next, I?d have a close look at the 100 panel. Is it labeled? Do you believe the labels? Can you tell what equipment is on Phase A? If so, I?d look into the equipment that does not operate constantly, but rather is operated once or twice a week.
Re: Fuses keep blowing
Also, doen't forget to check the fuse clips. A faulty fuse clip can cause heating to the fuse. Since a fuse protects with a thermal element you can conclude what happens should the clips and/or wire terminations cause heating to the fuse, that is to cause the fuse element to melt prematurely.
Also, doen't forget to check the fuse clips. A faulty fuse clip can cause heating to the fuse. Since a fuse protects with a thermal element you can conclude what happens should the clips and/or wire terminations cause heating to the fuse, that is to cause the fuse element to melt prematurely.
Re: Fuses keep blowing
It sounds like you may have a cable with insulation failure. You may want to megger each cable to see. If the voltage is 120/240 the voltage is not high enough to sustain the arc.
If you megger the phase conductors you should be able to do a compaarison to determine if you have a insulation problem.
[ August 22, 2003, 01:39 PM: Message edited by: bob ]
It sounds like you may have a cable with insulation failure. You may want to megger each cable to see. If the voltage is 120/240 the voltage is not high enough to sustain the arc.
If you megger the phase conductors you should be able to do a compaarison to determine if you have a insulation problem.
[ August 22, 2003, 01:39 PM: Message edited by: bob ]
pierre
Senior Member
- Location
- Westchester County, New York
Re: Fuses keep blowing
Expanding on what Charlie mentioned about the possible groundfault. If the ground return path is of a high impedance, the amount of fault current during the ground fault will not trip the fuse immediately, but possibly at the 3 day delay based on the time curve of the fuse and the amount of current actually reaching the fuse. I would check for a groundfault along the circuit, this may not be easy. One thing you could check for is no current on the ground path when the fuse is blown, and/or current on the ground path when the fuse is replaced.
Good Luck
Pierre
Expanding on what Charlie mentioned about the possible groundfault. If the ground return path is of a high impedance, the amount of fault current during the ground fault will not trip the fuse immediately, but possibly at the 3 day delay based on the time curve of the fuse and the amount of current actually reaching the fuse. I would check for a groundfault along the circuit, this may not be easy. One thing you could check for is no current on the ground path when the fuse is blown, and/or current on the ground path when the fuse is replaced.
Good Luck
Pierre
Re: Fuses keep blowing
Some good advice. One of you asked what kind of store it is. It is a convenience store with a walk in beer cooler and 3 condensors on the roof. They have 2 peices of cooking equipment that are plugged into 120V outlets, a 240V ice machine, a 120V freezer for bags of ice, 2 - 120V refrigerators, and a 240V water heater.
Another person asked what type of fuses were used, they are time delay fuses.
Some good advice. One of you asked what kind of store it is. It is a convenience store with a walk in beer cooler and 3 condensors on the roof. They have 2 peices of cooking equipment that are plugged into 120V outlets, a 240V ice machine, a 120V freezer for bags of ice, 2 - 120V refrigerators, and a 240V water heater.
Another person asked what type of fuses were used, they are time delay fuses.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Re: Fuses keep blowing
I would start by utilizing the FOP (fall of potential) test method on each pole of the FSS line to load and narrow the investigation fuse clips knife blades etc.
Generally (NOT ALWAYS) faults once they appear either burn clear (not normally) or get worse and finally become a constant failure.
Meggering the cable would be step two and step three would be to utilize a recording ampmeter.
I would start by utilizing the FOP (fall of potential) test method on each pole of the FSS line to load and narrow the investigation fuse clips knife blades etc.
Generally (NOT ALWAYS) faults once they appear either burn clear (not normally) or get worse and finally become a constant failure.
Meggering the cable would be step two and step three would be to utilize a recording ampmeter.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Fuses keep blowing
The comment by templdl (a good suggestion) brought another thought to my mind. Can you beg or borrow (I wouldn't suggest stealing) a thermal imaging test device? An infrared image of the fuse box (or other locations in the store) may reveal the problem.
The comment by templdl (a good suggestion) brought another thought to my mind. Can you beg or borrow (I wouldn't suggest stealing) a thermal imaging test device? An infrared image of the fuse box (or other locations in the store) may reveal the problem.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Re: Fuses keep blowing
Charlie B:
Utilizing the FOP test procedure negates (for a one time view of a not so complex piece of distribution equipment) the need to IR, though non contact thermometers are a valuable tool for items such as this.
Charlie B:
Utilizing the FOP test procedure negates (for a one time view of a not so complex piece of distribution equipment) the need to IR, though non contact thermometers are a valuable tool for items such as this.
charlie tuna
Senior Member
- Location
- Florida
Re: Fuses keep blowing
i am a certified thermographer and would like to recommend a scan of this system--but - if you have a constant load of say 60 amps it really doesn't take but two 20 amp circuits to blow the fuse... forget the breaker on the downstream panel - the fuse will always open faster than the breaker. to really determine whats happening here is to place a recording data recorder on the line side of this fused disconnect switch. if the fuse blows without a high current reading on the recorder would tell you a connection problem in the switch..
i am a certified thermographer and would like to recommend a scan of this system--but - if you have a constant load of say 60 amps it really doesn't take but two 20 amp circuits to blow the fuse... forget the breaker on the downstream panel - the fuse will always open faster than the breaker. to really determine whats happening here is to place a recording data recorder on the line side of this fused disconnect switch. if the fuse blows without a high current reading on the recorder would tell you a connection problem in the switch..
dan55
Member
- Location
- South Dakota
Re: Fuses keep blowing
There wouldn't happen to be a 120v water heater on the premises by chance? Seen a similiar problem before. 120v water heater added to big of a load on one phase. It was a single phase panel that was normally balanced pretty well till the hot water heater kicked in.
There wouldn't happen to be a 120v water heater on the premises by chance? Seen a similiar problem before. 120v water heater added to big of a load on one phase. It was a single phase panel that was normally balanced pretty well till the hot water heater kicked in.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Re: Fuses keep blowing
With any problem start with the easy and keep taking it another step, (my firm also employs Level Three certified thermographers, but I would rush one to the site for this problem). The FOP test is simple and can isolate, high resistance problems, will save time and money. If the millivolt drop is similar from pole to pole take it to the next level verify the loads, then check the distribution system for insulation failures, then monitoring equipment.
IR is always a recommend procedure for non-intrusive non-destructive maintenance program.
With any problem start with the easy and keep taking it another step, (my firm also employs Level Three certified thermographers, but I would rush one to the site for this problem). The FOP test is simple and can isolate, high resistance problems, will save time and money. If the millivolt drop is similar from pole to pole take it to the next level verify the loads, then check the distribution system for insulation failures, then monitoring equipment.
IR is always a recommend procedure for non-intrusive non-destructive maintenance program.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Re: Fuses keep blowing
, (my firm also employs Level Three certified thermographers, but I would NOT rush one to the site for this problem).
Not sure if it is my mind or typing skills but it should have said WOULD NOT.
, (my firm also employs Level Three certified thermographers, but I would NOT rush one to the site for this problem).
Not sure if it is my mind or typing skills but it should have said WOULD NOT.
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: Fuses keep blowing
Some of these machines have a heating element that runs periodically to evaporate the condensation build-up. Maybe it is something like this?
Re: Fuses keep blowing
One suggestion would be to purchase a Fluke Model 80T-IR Non-Contact Infrared Temperature Probe. It may pay for itself in a couple of uses when heating is a suspect. I have one that I keep with my Fluke 26 which I use when I suspect a temperature related problem. I was most impressed with the device when I troubleshot a 2p main breaker with a bad contact in one of the poles. The probe indicated a noticably higher temperature on one of the defective pole.
However, the the problem that one has is of being there at the time or very shortly after the even has occurred. A fuse that has blow because of a heating problem from the clips or terminals will cool off by the time you get notification of the problem. The same darned thing happens when you try to trouble shoot why a breaker tripped. If you aren't there at the time of trip of immediately thereafter it's basically imposible to tell how it tripped. If the breaker tripped thermally it most likely can not be reset intil it cools off some. A breaker that trips instantaneously can be reset and closed immediately althout closing a breaker back into a fault is not the most wisest thing to do.
One suggestion would be to purchase a Fluke Model 80T-IR Non-Contact Infrared Temperature Probe. It may pay for itself in a couple of uses when heating is a suspect. I have one that I keep with my Fluke 26 which I use when I suspect a temperature related problem. I was most impressed with the device when I troubleshot a 2p main breaker with a bad contact in one of the poles. The probe indicated a noticably higher temperature on one of the defective pole.
However, the the problem that one has is of being there at the time or very shortly after the even has occurred. A fuse that has blow because of a heating problem from the clips or terminals will cool off by the time you get notification of the problem. The same darned thing happens when you try to trouble shoot why a breaker tripped. If you aren't there at the time of trip of immediately thereafter it's basically imposible to tell how it tripped. If the breaker tripped thermally it most likely can not be reset intil it cools off some. A breaker that trips instantaneously can be reset and closed immediately althout closing a breaker back into a fault is not the most wisest thing to do.
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