Intermittent problem blowing fuses

[sparky 134]

I'm pulling my hair out over this one. Customer is a golf course with an irrigation control cabinet set in the middle of the course. POCO transformer is located next to the cabinet and provides a 480volt 100amp 3phase Bphase grounded service. Power enters the meter on the back of the cabinet, then to a 3R disconnect, then nipples into the control cabinet which was manufactured years ago by a vendor. Power enters a disconnect switch, load side of switch feeds a distribution block which is tapped to feed three 100amp 600volt fuses and tapped to feed two 15amp 600volt fuses. The 100amp fuses feed a motor starter which in turn powers an irrigation pump. My problem is with the 15amp fuses.

The two 15amp fuses feed a 480volt to 120/240volt dry type transformer which is located outside the irrigation control cabinet but within the overall cabinet. Let me explain, there is a large 5'x5' 3R cabinet and inside this cabinet is the irrigation control panel, transformer and other miscellaneous equipment . There is a sealtite whip from the irrigation control panel to the transformer and it contains both the primary and secondary conductors. The secondary conductors terminate to two 30amp 250volt fuses and these feed a small Square D QO load center.

The 15amp 600volt primary fuses keep blowing intermittently. The load on them is only 2.5amps. The load on the secondary fuses is only 5amps. This problem first surfaced two years ago and I replaced the transformer and surge protection device and the problem went away. Now it's back. I thought maybe one of the branch circuits which feed various equipment was shorting so I went ahead and installed small 5amp fuses on each of the branch circuits hoping one of them would blow and give me a clue. No such luck, none of them blew but the 15amp fuses did.

The transformer is tapped into the A and C phases of the incoming power. I'm at a loss as to my next step. I've contacted the POCO and requested they install a PQ meter on the service to see if maybe there's an intermittent spike coming in and blowing the fuses. The fuses are a current limiting time delay style.

Any ideas ?

 

[petersonra]

do both fuses blow each time?

might be you have an intermittant winding fault.

have you tried meggering the transformer?

 

[jim dungar]

What size transformer?

What type of transformer (i.e. 'open style' control power)?

Can blown fuses be traced to a power-down then power-up event, like a utility 'blip'?

 

[sparky 134]

"do both fuses blow each time?" - Sometimes just one does, other times both do.

might be you have an intermittent winding fault.

"have you tried meggering the transformer?" - No, because this problem existed with the old transformer and the new one.

"What size transformer?" - 5kVA

"What type of transformer (i.e. 'open style' control power)?" - Standard dry type.

"Can blown fuses be traced to a power-down then power-up event, like a utility 'blip'?" - As far as I know the power is not being shut off unless the POCO is having trouble I don't know about.

The night time crew does their watering at night and this is when the problem happens. Sometimes the fuses blow right away, other times it happens after a couple hours of watering. The cabinet is not being sprayed with water. I sat there one night for a few hours with my Fluke meter and amprobe on the circuit and of course they never blew. The next night they did. After I "fixed" it in 2010 the problem disappeared for almost two years.

Could the 480volt B-phase being grounded and the transformer being grounded have anything to do with this ?

 

[__dan]

The stand alone utility service and irrigation controller/plumbing out in the middle of a golf course is a high probability candidate for repeated nearby lightning strikes. You could have been hit hard enough for insulation damage but not hard enough for visible damage.

The control transformer is probably one of the most fragile components in your arrangement, except for the stuff after the transformer. The 5 kVA seems kind of large for the starter control, unless you are running lights and convenience outlets off it. The numbers you gave indicate the control voltage is 240 ? 2.5 A @ 480 = 5 A @ 240 = 1.2 kVA. Maybe a cabinet electric heater also?

This is to save money on an inexpensive replacement part, if you can go smaller on the control transformer, 2 kVA? with 5 or 6 amp primary fuses. Smaller is better sometimes if it's a sacrificial device in front of lighting strikes.

If it's lightning damage, it will keep getting hit. On the primary side in front of the transformer, I would look for fuses, then the surge arrestor, with right sized fuses blowing easily for hits. Some SPD's have hit counters.

 

[sparky 134]

The stand alone utility service and irrigation controller/plumbing out in the middle of a golf course is a high probability candidate for repeated nearby lightning strikes. You could have been hit hard enough for insulation damage but not hard enough for visible damage.

The control transformer is probably one of the most fragile components in your arrangement, except for the stuff after the transformer. The 5 kVA seems kind of large for the starter control, unless you are running lights and convenience outlets off it. The numbers you gave indicate the control voltage is 240 ? 2.5 A @ 480 = 5 A @ 240 = 1.2 kVA. Maybe a cabinet electric heater also?

This is to save money on an inexpensive replacement part, if you can go smaller on the control transformer, 2 kVA? with 5 or 6 amp primary fuses. Smaller is better sometimes if it's a sacrificial device in front of lighting strikes.

If it's lightning damage, it will keep getting hit. On the primary side in front of the transformer, I would look for fuses, then the surge arrestor, with right sized fuses blowing easily for hits. Some SPD's have hit counters.

There is a surge protector on the incoming 480volt power and there is one on the small load center. Both of them are still showing green indicators. The transformer provides power for two remote watering station control panels, the pump control circuit, cabinet mounted GFCI receptacle and a couple of other ancillary pieces of equipment. I replaced the transformer with the same size as the original one.

My next step may be to megger the transformer.

 

[jim dungar]

I would be more likely to blame the SPDs before the transformer. In my experience, transformer problems, other than during inrush, are rarely sporadic.

 

[Cow]

I once had trouble with a 10kva transformer that kept blowing fuses. The machine it was on had power cycled to it all the time, it was inrush that was blowing the fuses.

I'd recommend megging the transformer and then installing 25 amp cc fuses on the primary side.

 

[brian john]

It is either a fault or thermal.

Megger and FOP

 

[sparky 134]

It is either a fault or thermal.

Megger and FOP

I thought about a thermal problem but the load on the transformer doesn't change when they are watering. What is FOP ?

 

[masterinbama]

Are you sure it's not a fuseclip to terminal block issue? Had one one time that would show no voltage on the load side of one fuse. I would turn the power off, replace the fuse and it would work for a day or so. after replacing 6 or so fuses without being able to find a fault, short or the like I was at my wits end. Come to find out that it was the connection between the fuse clip and the line lug that was loose. Pulling and replacing this fuse would reestablish contact for anywhere from an hour or so to a few days, tightened the screw and all was well.

 

[sparky 134]

Are you sure it's not a fuseclip to terminal block issue? Had one one time that would show no voltage on the load side of one fuse. I would turn the power off, replace the fuse and it would work for a day or so. after replacing 6 or so fuses without being able to find a fault, short or the like I was at my wits end. Come to find out that it was the connection between the fuse clip and the line lug that was loose. Pulling and replacing this fuse would reestablish contact for anywhere from an hour or so to a few days, tightened the screw and all was well.

I meter the fuses and they are indeed blown.

 

[brian john]

FOP Fall of Potential a test method utilizing a digital multi-meter (DMM)

I wrote this a while ago.

We are often called by electrical contractors to investigate why fuses are blowing or CBs are tripping. Many times the cause is a high resistance connections resulting in sufficient heat to effect the thermal element in the fuse or CB.

A simple method to isolate the high resistance connection and thus the source of the heat is the Fall of Potential Test Method, commonly referenced to as the FOP test. To perform this test, one simply needs a multimeter with a millivolt scale, and an amp clamp.

There needs to be a load on the device to be tested, preferably a balanced load or close to balanced load. In the case of a fused safety switch (FSS). One would measure current across all three phases, then measure from line to load of one pole/phase of the conductor strands (if exposed) for each pole of the FSS. If one phase has a higher that average millivolt measurement (actually the voltage drop across the device under test). Your next measurement would be from line conductor to line of the fuse, if all readings are close to equal move to the next components of the FSS, in this manner you an isolate the high resistance connection.

With an arranged outage repairs can be implemented and a repair FOP measurement taken to verify repairs.

Our thermographers perform this test as part of their IR Scan to isolate to high resistance issue. As sometimes it is not possible to determine from a picture if the issue is a CB connection to the bus or the CB. Additionally it is not feasible to use a DLRO (Digital Low Resistance Ohm Meter)/ Micro ohm-meter to take measurements on small CBs and FSS due to contact point spacing of the test instruments, so our technicians take pre-repair and post-repair measurement s to verify repairs.


An example we IR?d a 200 amp CB this weekend with 155 amps per phase (average), millivolt readings were 38mv, 91mv and 42 mv. The readings were taken from the bus stabs of the CB, negating any possible issue with the CB to bus connection or conductor termination connector to CB connection. B phase had an issue, when we replace the CB we will do further testing and open the CB to see if visual thermal damage has started.

This test can be performed on single pole CB, or any 3-pole devices, we have used this on 4000 amp bolted pressure switches.

As with any testing of exposed energized parts, all safety cautions must be observed, wearing of PPE, isolating the area to be worked in. One issue we have had over the years is customers taking FLASH photography as we are taking measurements. We no longer permit customers to take photos, without prior notice. This minimizes heart attacks
__________________
Brian John
Leesburg, VA

 

[hillbilly1]

While you are meggering the transformer, I think I would Megger the wires from the fuses to the transformer that are in the sealtite. You may have hidden lightning damage to the insulation there.

 

[brian john]

While you are meggering the transformer, I think I would Megger the wires from the fuses to the transformer that are in the sealtite. You may have hidden lightning damage to the insulation there.

If you are going to megger for a possible short, you need to megger all components of the distribution system that are downstream of the tripped CB or blown fuses.