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Overheated lugs and blown fuses 2

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sferrari

sferrari

Industrial
Jul 14, 2021
9
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.
 
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Overheating could have been occurring for a long time. Maybe the root cause was an intermittent fault in the motor terminal box. Or the autotransformer starter is not operating correctly.

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.
 
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I was told the motor was meggered from the load side of the starter. I should have confirmed.

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.
 
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"The equipment is pretty old and worn out. The fuse holders/lugs/wires for the blown fuse had clearly overheated."

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.
 
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If it's autotransformer based there should be 3 contactors.
 
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Compositepro - The equipment has NOT operated without problems. The inside of the starter panel looks like a war zone.

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.

 
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Fuses are thermally actuated based on melting the fuse material. The usual source of heat is the tiny resistance of the fuse which creates the heat. When it is overloaded that source of heat exceeds the rate at which heat can leave. However, if the fuse is externally heated so there is not a good path out, ordinary loads may be enough added heat.

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.
 
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What 3DDave said.

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 -
 
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Once there is a little heating it causes thermal cycling which accelerates the damage and the heating gets worse.
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:
image_kf3lcz.png


Range of Sizes
image_uceiyh.png


--------------------
Ohm's law
Not just a good idea;
It's the LAW!
 
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All of this should be fairly measurable with a volt meter across the fuse holder. Then again, this doesn't sound like the kind of cabinet you want to be probing around in live.
 
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All of this should be fairly measurable with a volt meter across the fuse holder
Click to expand...
Been there, done that. With a couple of hundred amps, less than one Volt drop across a fuse holder or switch blade will cause serious heating and damage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!
 
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It should be possible to establish a baseline of what is "normal". 50 watts is a pencil heater for an enclosure so maybe that's a good starting point (those are about the same size as a fuse holder and get roasting hot). At 400 amps 50 watts of heating would be about 0.1 volts across the fuse (enough to cause significant heating). A mV meter should be able to give you values below that 0.1V to give you an idea of the quality of connection. The fuse itself will create some voltage drop but I would expect to see drops in the low 10's of mV.

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
$650 for the highest featured model is a lot less than anything coming from Fluke.
 
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Been there, done that.
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!
 
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Your run-of-the-mill temp gun will even get the job done. Probably anything over 120F would be a problem unless someone has a better temp suggestion.


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 -
 
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Overheated fuse clips preheat the fuse so that it opens under lower than expected current. That's an easy call. If the connection is bad enough, the fuse will open under normal current. If that's the case the overheating should be evident due to discolored and oxidized metal.

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
 
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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
$650 for the highest featured model is a lot less than anything coming from Fluke
Click to expand...
I'm intrigued since I'll be needing to replace my personal Android phone soon.

Best Buy link

spec sheet

unlocked - check. 6gb ram check. 128gb rom - check.
The operating system is listed as Android 10 (Go edition). As far as I understand, Go edition is used in 3rd world countries, not widely in the US as far as I understand. The operating system and security update schedule are my questions. If I can resolve those I'd be really interested in this (I like the hardened case along with the thermography).

=====================================
(2B)+(2B)' ?
 
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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.

DO NOT ACTUALLY LICK ANY ELECTRICAL ITEM, PLUGGED IN OR NOT, RED HOT OR NOT. NO LICKING.

I feel like I need to add that for liability reasons.
 
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Damn. I needed this today.

Lots of good info and some funny stuff.

Everyone's input is greatly appreciated.
 
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Dear Mr. sferrari (Industrial)(OP)
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)
 
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