Disconnect Melting

J

JoelHunter1102

Member
Location
San Diego
Occupation
Electrician
Good afternoon all,
I have a situation that is stumping me.

My coworker installed a disconnect (100A 3ph 208v) for a client and the disconnect had the C leg melt, without blowing the fuse. So we replaced it, it happened again. Replaced it for a 3rd disconnect, and this time upsized the disconnect to a 200A heavy duty disconnect, and used fuse reducers to put new 100A fuses in.
Lo and behold, it lasted longer than the others, but ALSO melted the C leg without tripping the fuse... it took a couple of years to do so, but still ended up with the same result.

After the 1st time we ran a bunch of calculations and made sure wire sizing/distance/etc was all good. After the 2nd time we hired an outside engineering team to come in and do some studies, they deemed everything was fine. We have monitored it ourselves and keep coming up with the equipment pulling about 68A per leg...so not enough to trip the fuses, but somehow still melting the disconnect.

We assumed weak internal connections in the disconnect itself (faulty from the factory), but at this point after three disconnects met the same end, my coworker and I are a bit dumbfounded.

Anyone have any thoughts?

Thanks in advance!
 
Which lugs or fuse clips are failing, the line or the load?
What is your system, how many wires, what voltages?
 
What's the connected gear? Sounds to me like a harmonics issue of some kind since your load is within spec, multiple disconnect replacements (plus the upsized one) pretty much rules out a factory defect. External heat sources seem unlikely since it's always C phase.
 
Tyler C said:
Potentially an external heat source is located to the right side of the disconnect, so it always melts C.
Click to expand...
It would have to substantially hot to melt the C phase components versus melting just the fuse. They have been replacing the entire switch.
 
ptonsparky said:
...
I would be inclined to think poor termination techniques. No torque wrench applied. Did they cut the damaged conductor back to good? Or reuse?
Click to expand...

I'm also thinking that the conductor is likely causing the problem. If they cut off the obviously damaged conductor, perhaps there is oxidation or other corrosion that goes further up the conductor. Is it copper or aluminum?
 
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A preventive monitoring option may also tried, IR scan instead:
A simple wireless temperature probe or IoT thermocouple setup be mounted inside the disconnect enclosure
monitor continuously critical hot spots, log data, and automatically send alerts to a smartphone or computer when a set temperature limit is reached, cost-effective preventive option for repeated disconnect overheating.
 
Good morning guys,
Sorry for the delay, i wanted to get the answers to your questions from my coworker (he is the one who has swapped it out each time), since he has been more directly involved.
So, answers below, in order of responses:


1. Retirede - I would hope its not sabotage, this is at an indoor shooting range, and i don't see why they would benefit from losing a whole lane when this happens until we can fix it.

2. LarryFine - We hired an outside company (to help ensure the customer we were not just being shady and unwilling to admit it might be our mistake) to do a thermal inspection. They were the ones who said everything looked fine. After discussing with them, it was decided we should try and upsize the disconnect. Which we did, bought us more time...but still eventually had the same result.

3. Tyler C - No external heat sources nearby that we are aware of. It is on a mezzanine and feeds a fan, but the fan motor is 15 feet away.

4. jim dungar - System is 208v 3ph. Only one phase is having this issue, and i was incorrect in my initial post; it is the B phase and it is the line side of the disconnect.

5. Modbusman - What do you suggest if it is a harmonics issue? The whole building is 208v 3ph, there are no transformers after the main switchgear....if that helps.

6 & 7. Tyler C & jim dungar - Again, i apologize i was mistaken originally, it is the B phase that is being melted.

8. ptonsparky - We did a thermal test, even had an external company do a thermal test (to eliminate any potential conflict of interest in the results). Have not done FOP test, but that might be worth looking at also.
I will give it that maybe the first time may have been poor installation, but the 2nd and 3rd time we were pretty adamant about doing crossing all our T's and dotting our I's.
We did notice burn marks on the wire, so we cut back about 2' to get back to shiny copper.
I dont have before and after pictures at this time.... but i do have a picture of the latest melted disconnect....however since i only signed up for this forum to ask about this question, i apparently do not have enough posts to allow me to upload any pictures yet. Sorry :(


9. synchro - Conductors are copper. At this point I am not willing to rule out anything such as oxidization further up the conductor, but without replacing the entire run, we cannot verify. Which obviously we are trying to avoid.

10. Deltaforce - That is an interesting idea. We did monitoring on each leg and left that to run for a couple weeks, kept any eye on it and over the course it never went dangerously high on any leg.
Do you have an IR camera that you recommend to do what you are suggesting?



Thanks again everyone, all the input is helpful and valuable!
 
JoelHunter1102 said:
4. jim dungar - System is 208v 3ph. Only one phase is having this issue, and i was incorrect in my initial post; it is the B phase and it is the line side of the disconnect.
Click to expand...
I think much of what we have discussed may be modified by this one answer. The most important so far. You initial post talked about the fuses not blowing. That is misleading to many of us because the only way the fuses would have an impact is if you were on the load side of them. So, from there, we should start again. What can cause the heat:
  1. High amperage
  2. a high resistance at the connection.
  3. High surrounding temperature. External heat source.
  4. high heat on the wire transferring (wicking) along he wire to the termination
  5. high heat on the disconnect components transferring (wicking) to the termination.
#5 and #3 seem highly unlikely. 100% the first thing I would do is put a power monitor on the wire and also monitor voltage on the load side of the termination.

I will shut up and let others speculate on whether there are additional items that could cause the problem and what the solutions should be.
 
JoelHunter1102 said:
5. Modbusman - What do you suggest if it is a harmonics issue? The whole building is 208v 3ph, there are no transformers after the main switchgear....if that helps.
Click to expand...
Harmonics could be a thing if there were a lot of electronic equipment, but it sounds like what you're dealing with is essentially service-disconnect-line-connected fan, so that pretty much rules harmonics out as a possibility. That being said, I do wonder if maybe there's something wrong inside the fan itself, either a miswire inside the peckerhead, or perhaps a winding going the wrong direction. My experience with motors pretty much ends at the VFD, but the other guys here could almost certainly chime in as to whether or not this is a legit possibility, or me just wandering off the farm.
 
ModbusMan said:
Harmonics could be a thing if there were a lot of electronic equipment, but it sounds like what you're dealing with is essentially service-disconnect-line-connected fan, so that pretty much rules harmonics out as a possibility. That being said, I do wonder if maybe there's something wrong inside the fan itself, either a miswire inside the peckerhead, or perhaps a winding going the wrong direction. My experience with motors pretty much ends at the VFD, but the other guys here could almost certainly chime in as to whether or not this is a legit possibility, or me just wandering off the farm.
Click to expand...
Wandering...
 
ModbusMan said:
Harmonics could be a thing if there were a lot of electronic equipment, but it sounds like what you're dealing with is essentially service-disconnect-line-connected fan, so that pretty much rules harmonics out as a possibility. That being said, I do wonder if maybe there's something wrong inside the fan itself, either a miswire inside the peckerhead, or perhaps a winding going the wrong direction. My experience with motors pretty much ends at the VFD, but the other guys here could almost certainly chime in as to whether or not this is a legit possibility, or me just wandering off the farm.
Click to expand...
Also, since this is happening on the line side of the disconnect you can rule out something in the fan itself.
 
Strathead,
per your itemized list below:
Strathead said:
  1. High amperage
  2. a high resistance at the connection.
  3. High surrounding temperature. External heat source.
  4. high heat on the wire transferring (wicking) along he wire to the termination
  5. high heat on the disconnect components transferring (wicking) to the termination.
Click to expand...

1. We have put monitoring equipment on and tracked amperage for a couple weeks (maybe a month) at a time. It never pulled over 72 amps on any leg. Voltage never dropped or went haywire.
2. We are now on our 4th disconnect (numbers 3 & 4 are 200A HD disconnects for the 100A load). So not sure how high resistance would be a recurring issue across 3 disconnects at the same point.
3. High surrounding temperature is legit, it is above a gun range, outside of all HVAC cooled areas. But wouldn't that cause the issue to happen on more than just the B leg every time?
4. So you suggest maybe upsizing the conduit and wire to give more area for the conductors to breathe?
5. Not sure what would cause high heat to be focused on the line side of the middle leg of a disconnect.... in the picture i was sent it doesnt look like our conduit penetration is directly above.... which i now have the ability to attach pictures. so please see attached!


Thanks all!
 

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How often is this switch operated?

The heat is not at the conductors, so I would not focus there.
The damage is at the fuse clip and disconnect blade. This usually comes from using the switch as a motor starter which is why switches have HP ratings significantly lower than than their normal rating.

The B phase fuse, in the picture, does not appear to be fully seated in the fuse clip, when compared to the C phase. Are the fuses being replaced each time the switch is?
 
jim dungar said:
The B phase fuse, in the picture, does not appear to be fully seated in the fuse clip, when compared to the C phase. Are the fuses being replaced each time the switch is?
Click to expand...
Just reading this thread and was also wondering about the fuses. Are they are being replaced or the same fuses being used and on the same legs each time.
 
jim dungar said:
How often is this switch operated?

The heat is not at the conductors, so I would not focus there.
The damage is at the fuse clip and disconnect blade. This usually comes from using the switch as a motor starter which is why switches have HP ratings significantly lower than than their normal rating.

The B phase fuse, in the picture, does not appear to be fully seated in the fuse clip, when compared to the C phase. Are the fuses being replaced each time the switch is?
Click to expand...
I agree, that fuse is not seated properly.
 
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