Overheated lugs and blown fuses
Overheated lugs and blown fuses
(OP)
A couple of days ago I was on a service call on a 250hp 3p 480vac agricultural well that blew a 400A fuse. The fused disconnect feeds an autotransformer soft starter. The equipment is pretty old and worn out. The fuse holders/lugs/wires for the blown fuse had clearly overheated. I cleaned the corrosion as well as I could, installed the fuse and powered up the system. Voltage and current balance were good. I'm told that it only ran for 2 hours before the fuse blew again. When it blew, it also overheated/shorted one of the terminations in the peckerhead.
We got a new bucket and fuses. We're replacing it and the conductors between it and the starter tomorrow. I won't be there because I need to be somewhere else and the electrician that will be there isn't inclined to troubleshoot. He's just gonna swap the parts.
Could the generally crappy condition of the conductors/fuse holder be the cause of blowing fuses? If so, could someone explain what's happening there?
I REALLY don't want this problem to persist after our electrician finishes tomorrow. Any help would be greatly appreciated.
We got a new bucket and fuses. We're replacing it and the conductors between it and the starter tomorrow. I won't be there because I need to be somewhere else and the electrician that will be there isn't inclined to troubleshoot. He's just gonna swap the parts.
Could the generally crappy condition of the conductors/fuse holder be the cause of blowing fuses? If so, could someone explain what's happening there?
I REALLY don't want this problem to persist after our electrician finishes tomorrow. Any help would be greatly appreciated.
Talk To Other Members
RE: Overheated lugs and blown fuses
Someone should megger the motor, then megger the circuit from the starter to the motor. I would disconnect the motor, then energize the starter to see if it times out and transitions properly to full voltage.
RE: Overheated lugs and blown fuses
Please elaborate on the autotransformer not operating correctly. One of the conductors on the "load side" of the autotransformer was severely corroded and the rest looked like they were lightly toasted.
I made sure to confirm that the motor switched over from the start contactor to the run contactor. That isn't to say it didn't switch back to the start contactor, close both contactors, open both contactors, alternate back and forth rapidly, and then open and close both of them rapidly as soon as I left. This panel is really clapped out.
Thanks for the reply.
RE: Overheated lugs and blown fuses
That pretty much says it all. The equipment has operated a long time without problems. A poor contact at the fuse holder will create heat and cause the fuse to melt. Degradation of electrical contacts is a rapidly self accelerating process, once it starts.
RE: Overheated lugs and blown fuses
RE: Overheated lugs and blown fuses
Heat/damage to a fuse holder creates resistance. Does the resistance make that leg draw more current? Enough to blow a 400A fuse?
Tugboat - I've seen schematics for autotransformer soft starters with 3 contactors. I've never seen one in the real world. They all have two, in parallel, start contactor goes through autotransformer, and the other is DOL.
RE: Overheated lugs and blown fuses
I would expect that the unit worked for a notable amount of time after the fuse was replaced rather than instantly when the circuit was closed. This would match the time required for the external heat to build enough to cause the fuse to fail.
There may be additional current as the voltage drop through the connection decreases the voltage applied to the motor which, in turn, will draw more current. I don't know that the amount of increase is a critical part of the problem as similar voltage drops occur during brown-outs and don't usually cause fuses to blow.
RE: Overheated lugs and blown fuses
I've seen more than 10 cases where fuses blew without overload due to the fuseholders heating up so the fuse's normal steady state heating added to the smoking hot fuse holder blows the fuse(s). Often you can follow that up with some serious single-phasing of the motor - likely the cause of your peckerhead problem.
A corroded in ANY WAY fuse holder should always be replaced as it is by far cheaper than multiple service calls or a single phasing.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Overheated lugs and blown fuses
Many years ago I worked in a plant with fused disconnects in all of the MCC buckets.
We found that the fuse holders tended to lose tension and start to run hot.
These helped a lot with weak fuse holders.
Bussman NO.7 Tron Clip Clamp:
Range of Sizes
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
RE: Overheated lugs and blown fuses
RE: Overheated lugs and blown fuses
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
RE: Overheated lugs and blown fuses
Or, since fuses and holders can get spendy in that amp range, use this as an opportunity to get a FLIR infra-red camera. Caterpillar used to include one in their branded cell phone. They're quire handy for this type of work.
Edit:
And they still make them https://www.catphones.com/en-us/
$650 for the highest featured model is a lot less than anything coming from Fluke.
RE: Overheated lugs and blown fuses
When you measure from the fuse clip to the fuse or from the fixed contact to the movable contact on a disconnect and read a few hundred mili-Volts, then you have trouble.
I became aware of a problem when I was walking through a boiler room and felt the heat radiating from the main disconnect switch, three or four feet away. As I recall, the load was around 200 Amps or less, but four or five locations were showing around 300 mV drop.
If I get a chance to look at my old computer I may be able to find the file, but no promises.
A bad connection on the cable lug connecting to a fuse holder or disconnect switch may also initiate a heat failure.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
RE: Overheated lugs and blown fuses
My refrigeration buddy went into a dark hell-hole and as he was shuffling along a line-up of machines looking at his feet when he got to one machine his shoes were lit up by a glow. It was an overheating fuse holder.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Overheated lugs and blown fuses
Megging a motor from the starter? Again - easy call - if you get a satisfactory reading, you're done. If not, you've wasted five minutes and still have to break the connection at the motor. You didn't lose much time by doing the first meg reading from the starter. Yes, you're not getting an accurate reading of the motor, but this is just troubleshooting - is the circuit good enough to re-energize.
old field guy
RE: Overheated lugs and blown fuses
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RE: Overheated lugs and blown fuses
note this breathtaking comment from that link:
As an electrical engineer, you should probably lick it just to check for sure. If it tastes like cooked meat, you're all good.
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RE: Overheated lugs and blown fuses
Lots of good info and some funny stuff.
Everyone's input is greatly appreciated.
RE: Overheated lugs and blown fuses
1. There are numerous valuable/practical advice. ALL pointing to the fuse clip (failure) is the main cause of the problem. This is also my observation/experience in the field for the past 55+ years. There is no improvement for the past 100+ years.
2. I would strongly recommend to replace all fuses by MCCB. Bad contact on the MCCB main poles do happen, but hardly. Single-phasing would hardly happen at all.
Che Kuan Yau (Singapore)