Damaged lead on contactor

[SolarPrestige]

Looking at the attached photos, what's your first thought about what has failed? This cell set off an aspirated smoke detection early alarm. Heat damage (bad connection) or arcing damage (escaping from contactor) or something else?

 

[waross]

I have learned a lot on Eng-Tips by reading other people's posts, even when I don't at first agree with them.
By far the majority of the bad connections I have encountered have been due to improper pre-cleaning or not enough contact pressure.
Often both dirty and loose but it is sometimes hard to tell.
I can't remember problems with overtightened connections or any problems from lubricated threads.
If it doesn't break, bend or strip, it's not too tight.
I have seen too many devices rendered unusable due to thread stripping or galling that could have been avoided by a little lubrication.
This applies to circuit breakers, ground strips, neutral buses bus bar extensions and all of the devices that I have pictured.
Electrical manufacturers torque specs are often very light compared to general torque specs for the same size hardware.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. waross
W1. "...I can't remember problems with overtightened connections or any problems from lubricated threads.
If it doesn't break, bend or strip, it's not too tight."
C1. I repeat Please share with us your learned opinion with your decades of field experience which Standards/book that recommends to apply the conduction paste (on the thread) " as a thread lubricant on the bolt ".
C2. We (you and me) need a few more decades in the field.
C3. I repeat " Please share with us your learned experience if you have any different opinions on the points raised in my (four) posts."
Che Kuan Yau (Singapore)

 

[LionelHutz]

che12345 - Lubricant on lug threads can make a big difference. It really works. Doesn't matter if it's in a book or not.

Bill - The lug discussion is way off topic.

Both - I suggest dropping it and moving on. It's not contributing anything useful to the issue posted.

 

[LionelHutz]

I have been dealing with a project using that type of braided conductor. The manufacturer has charts that give the current ratings and I find their recommendations to be really light compared to what is normally used for wire sizing in North America. It's possible that at full current the heat rise is enough that the extra boot over the end is pushing the heat above the insulation limits.

Other than that, try better bolts and washers. The project I'm dealing with is using washers at least a 1/4" thick which cover as much of the end of the braid as possible.

 

[waross]

Don't limit yourself to electrical texts.
Look at instructions for general mechanical assembly.
One interesting site cites more failures due to undertightening than from over tightening.
Searching the web you will find many references to thread lubrication and anti-seize.
My "learned opinion with your decades of field experience" includes much repair of older equipment as well as new installations.
Penetrox is a good compromise for both thread lubrication and anti-seize.
By the way, Penetrox is not a good choice of lubricant for moving parts.
It is best used for connections and threads that are not expected to move in normal service.
In my "decades of field experience", I have occasionally encountered good quality equipment with aluminum alloy threads prelubricated with a paste type lubricant.
I didn't see that in a book.
You have a strong attitude of disrespect towards my ability and experience.
If you do not wish to accept information outside of the boundaries that you have set for yourself, then don't.
Just ignore me.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SolarPrestige]

A couple of months later, I have gotten hold of the contactor from the photos at the top. It is now easier to see the centre pole at the end that was getting the most damage. The electrician that was repairing this thought that an arc was escaping from the contactor and hitting the cable connection.
You can see in the photo that there is a dark screw with verdigris underneath it. The other two poles are clean and the screws are bright. Also the bolt head face that is turned toward the contactor has some sort of burning.
The arc must be pulled between two arcing plates and then pulled or pushed into the arcing chamber area. These are seen in the second photo. It does seem incredible that the arc plasma could get out, and I am inclined to think it isn’t. I think that the marking on the bolt might be burnt PVC from those boots that were sitting over the terminals. I can’t see any evidence of an arc tracking from underneath the cover out to the bolt head.
So maybe the dark screw is due to the same high temperatures. I expect that the centre pole gets hotter than the outer two poles. I might have to disappoint the electrician with a report debunking his theories.
What else could be going on here?

 

[itsmoked]

I'd suggest that there is not enough air flow. Everything else looks pretty reasonable with spacings and a pretty robust design. Too little air flow to carry away the inevitable heat is going to leave the center leg hotter. With enough air flow that asymmetric temperature rise won't happen.

Check the system's cooling system for issues:
1) Wrong filter element
2) Bad fan
3) Interfered air flow (clogged filters, something external too close to the inlets/outlets.
4) Subtle internal air flow blinding due to some up-wind device
5) Flat-out wrong fan for the application. Upgrade to a higher flow fan.

I'd check 1-4 and if nothing apparent is found execute 5) going for a 25~50% greater fan rating.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A strong possibility is a faulty cable.
If the stranded cable was too long in storage and then crimped without any measures to remove surface oxidation from the small strands, the heat may be generated internally to the crimp.
Also it may be well to remove the contact and check for signs of a heating on the bottom of the contact.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[EdStainless]

As Lionel said, go back and look at the voltage/current/temp ratings for these when connecting components (not just bus).
These straps have some fairly serious limitations.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[SolarPrestige]

Thanks for your thoughts. Do you guys have these motor control centres with cooling fans installed? We don't normally. An exception is for a section carrying a variable speed drive, but a plain old CB/contactor/overload cell (or electronic motor relay as this one has) never has cooling installed. The room is cooled with air conditioning units so its always nice on a hot day in there, and cold on a cold day. But the panels cool by radiating heat mostly, and a small amount of natural air flow through the gaps in the steel panel sections.

 

[itsmoked]

Do the units feel warm to the touch after a couple of hours running?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

With the exception of the VFD buckets as noted, fans in an MCC are a rare exception.
Looking at the photos again, particularly the fixed contact assembly, Does the fixed contact assembly show any signs or more heat than the other poles?
I have seen induced heat in one area cause extreme heat damage several inches away, without damage closer to the source of the heat.
A poor connection of the fixed contact may be causing conducted heat to heat up the cable strands.
Those fine strands are more susceptible to heat corrosion than are solid conductors, and the heat damage may be progressive.
Dismount the fixed contact and check for a poor connection to the base piece.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SolarPrestige]

The cell fronts would definitely feel warm after some running hours. I can't see any extra heat effects on the fixed contact, centre leg, or the moving contact. The impression of extra heat is on the bolted connection area of the centre white phase.


Waross, these flexible leads are factory manufactured, the ends are forged under high pressure. Not sure exactly what they do, it may include a copper sleeve over the stranded wire, but the copper at the ends is looking like a solid mass as we receive them.

When I am next working from work, I will unscrew the fixed terminal and have a look underneath it. Currently at home avoiding too much human contact (Covid 19).

 

[waross]

Thanks for the up date.
Waiting for the next report.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

One consideration worth noting, that I've come across in a few installations, is that the maximum temperature of the terminals of some of those contactors is stated as 75 degrees in manufacturer's documentation.
However, most of the standards and calculations regarding cable capacity use either 90 or 110 degrees as the maximum conductor temperature. 75 degrees is not uncommon for some of the PVC insulated cables in use down under, but for most industrial installations this is ignored as its generally not an accepted practice to use the lower rated cable.

Noting that the photographs show overheating in the internal connection section under the cover, not just where the lug on the Cu-Flex connects to the contactor, it'd be worth reviewing the design documentation to see if there isn't a limiting temperature for the contactor terminals.

 

[che12345]

Dear Mr. SolarPrestige
" ... When I am next working from work, I will unscrew the fixed terminal and have a look underneath it. Currently at home avoiding too much human contact (Covid 19). ..." Have a nice day.
I noticed that there are 34 replies as to date. A subject of importance and interest.
I had submitted my opinion earlier and would like to add the following:
a) there are six terminations on the contactor presumably with the same conductor size, method of connection, including type of terminal lug, etc. Only one terminal is over-heated,
b) there are twelve contact points (fixed and moving). No sign of over-heating,
c) there must be another three connection terminals at the up-stream breaker/fuse and another three or six connections on the motor (depending on the method of starting). There is no over-heating reported,
d) it is likely that there are many more starters with connection carryout in the same method in the same MCC. No over-heating is reported.
Bottom line: it happens only on one of many terminations carried out in the same method.
Possible causes: poor workmanship, foreign partials in between contact surfaces, loose connection, not checked with proper torque tool, etc.
Che Kuan Yau (Singapore)