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?

 

[itsmoked]

Either that center one was loose or being hemmed in on top and bottom led to it running much warmer. Slowly it oxidizes increasing its resistance > voltage drop > temperature > around-and-around until it toasts.

The typical causes in order are:
1) Fan died - middle cable wins the race to thermal hell.
2) Bolt was loose - excessive out-of-spec heating.
3) Unit over utilized drawing maximum for extended period(s).
3) Design flaw - under designed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

The 3rd "3" for Keith's list...

Flexible braided grounding straps as conductors, not designed for this use, covered with heat-shrink tubing to attempt to make insulated conductors out of them. In a previous life, we attempted something similar; using braided ground strap as conductors because they were more flexible and could bend in tighter spaces. But after repeated testing with UL, we could NEVER get it to pass the heat rise test. The extra fine stranded wire used to make the braided conductors don't conduct evenly through/to the connectors, leading to thermal runaway.

The alternative that worked was "Flexible Bus Bar" (a.k.a "Flexibus"), which allowed right angle bends (because it is bus bar, not wire) but was made from overlapping plates of thin bus bar and silver plating on each layer to ensure conductivity. Similar concept, better execution and designed for the task. The braided stuff is not.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

With either thin strips or with fine strands, it is almost impossible to remove any thin oxide layer that forms prior to making the final terminations.
It may be an issue when terminating used welding cables or cables that have been in storage for an extended time.
Locomotive cable, in kcm sizes, is composed of relatively fine strands.
Each strand is individually plated to avoid surface corrosion of the strands.

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

 

[SolarPrestige]

Thanks for comments. The black carbon I see is produced from the PVC boot degrading?
The leads are manufactured by Cubic as busbars made from braided copper and the ends are factory forged into solid pieces. We have a number of these used in starter cells. Most are OK but some have given issues over the past 10 years.

 

[waross]

If you have good results with the factory forged ends, then it is probably a loose connection, a dirty connection or a substandard factory assembly.

The black carbon I see is produced from the PVC boot degrading?

Typical copper heat corrosion.
You will see that even on bare bus bars that have been heat damaged by a poor connection, with no other material affected.

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

 

[waross]

Google "Copper oxide rectifier".
That black oxide of copper forms a rectifier.
The worst case that I have seen was a loose and badly heat corroded copper connection of a lead to a DC motor.
The motor would run forward, but when the current was reversed to reverse the motor the diode action prevented the motor from running in reverse.
(It wasn't running well forwards, but it would run forwards.)

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

 

[waross]

By the way, while not conclusive, equal oxidation on both sides of the forged end suggests a loose connection.
Copper oxide on the exposed side with the contact side relatively free of corrosion suggests a factory defect.

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

 

[SolarPrestige]

Cupric Oxide (CuO), a stable black solid formed by heating copper in air at 300 - 800 celcius. That's really good to know. Always something new to learn. I will see if I can find evidence of black on the connection side.
Other joints that have the same lead, and have given trouble, have some black on the contact side of the bolted end, but also other copper alloys with a light green colour. The photo shows one where the first concern was insulation falling off the lead, but looks like it was pretty hot as well.

 

[waross]

That looks like verdigris.
That is formed by exposure to moisture, high humidity, exposure to vinegar or exposure to sea air.
It has several chemical formulae depending on the cause of its formation.
I don't see obvious signs of heat.
Could the covering have been damaged by heat from an adjacent bad connection?

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

 

[Compositepro]

Once you overheat plastic insulation, there are plenty of corrosive compounds emitted.

 

[che12345]

Dear Mr. SolarPrestige

Q. " ....Looking at the attached photos, what's your first thought about what has failed? ...."
A. I looked into various invaluable opinions by regular learned contributors and would like to added the following.
1. Most likely due to loose connection, poor workmanship, dirty surfaces or tightened to lower than the recommended tightening torque.
2. The insulation is damaged by the heat generated likely > 100deg C at the poor contact point.
3. From the photos it appear:
a) that the hole size is rather large in comparison to the palm area. (Noted, that it is the standard type tested? product by the manufacturer).
b) it is evident that good electrical contact is limited to under the bolt head or nut, around the hole shown as a ring. Large area of the palm which is NOT making any electrical contact (air-gap) but over-heated shown oxidized, appeared in different colours.
3. Proposal:
a) chose the biggest bolt size suitable for the hole. Use grade 8.8 or higher bolts and grade 8 or higher nuts,
b) use bigger sized, thick washers under the bolt head and nut.
c) check the correct tightening torque recommended for the size of the bolt.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. SolarPrestige

Q. " ...Looking at the attached photos, what's your first thought about what has failed? ..."
A. Annexation to my post dated 2 Jul 20 11:37
1. 3 b) "... use bigger sized, thick washers under..." may be replaced by Belleville washers.
2. 3 c) "...check the correct tightening torque recommended for ..."
suggested guide line with + - 15% tolerance tightening torque in (Nm) for grade 8.8 metric bolt and grade 8 nut. M8 (28Nm), M10 (50Nm), M12 (86Nm), M16 (210Nm)
Che Kuan Yau (Singapore)

 

[SolarPrestige]

Bill, this lead with the verdigris on it is in a removable cell, installed in a motor control centre, and within a stand-alone building. Air issues could be from the sea as the wind can blow sea air our way, and/or cooling tower chemicals which we have seen affecting other parts of the plant. Seems the air can get in the MCC cells enough to cause this weathering. But the main issue with this lead was that the insulation was fragile and falling off. The adjacent leads were not as bad as this one. Could have been an insulation failure for some chemical reason. I asked the supplier/manufacturer to comment on this issue, but we didn't get a reply from them.

Thanks also Che, I am thinking about what we should do in future to keep these leads working well. We haven't been taking them apart to look at the connections, but may start doing this. Then we can clean and reassemble with better bolts and washers.

 

[waross]

Although it is intended for aluminum I have had good luck using Penetrox or No-Alox on copper connections and as a thread lubricant on the bolts.
When reconnecting a joint that has had heating issues, clean the mating copper parts to bright copper.
Don't be afraid to tighten the joints.
I don't know the reasons behind the manufacturers recommended tightening torque but I have more than once demonstrated that wires terminated in lugs tightened to the recommended torque can easily be pulled out.
Any movement in the wire tends to loosen them.

That verdigris is puzzling.
It doesn't look like heat damage, and verdigris doesn't need heat to form.
I see bright copper very close tho the termination and the discoloured strands look more the color of verdigris than the colour of heat corrosion.
That connection may still be in good shape despite the surface discolouration.
How does the other side of the connection look?

I have checked the condition of suspect joints by taking milli-Volt readings across the joint when under load.
With the same current flowing, the mV readings across a set of connections should be similar.
A failing connection will show a higher milli-Volt drop than the other connections.
The milli-Volt test is best done under the highest possible load.
The milli-Volt drop is proportional to the current.
If it is not possible to develop the same current across each phase, then scale the mV readings by the current ratios for comparison.

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

 

[che12345]

Dear Mr. waross
Q1. " ... Although it is intended for aluminum I have had good luck using Penetrox or No-Alox on copper connections and as a thread lubricant on the bolts..".
A1. a) Any commercial conduction paste intended for aluminium joints are usually NOT harmful to copper joints. It does NOT offer any usefulness/advantage either, but involved additional cost. This is evident that NO marine classification society rules/documents insisted that conduction paste shall be applied on all indoor dry copper joints.
b) the conduction paste is NOT intended to be used as a " thread lubricant on the bolts" . It would result to over-tightening!

Q2. "...I don't know the reasons behind the manufacturers recommended tightening torque but I have more than once demonstrated that wires terminated in lugs tightened to the recommended torque can easily be pulled out. Any movement in the wire tends to loosen them."
A2. a) the main reason for the manufacturers recommended (tightening torque) is to ensure that sufficient (pressure) is applied on the contacts. It is difficult/time consuming to measure the (pressure) or the (contact resistance) across the contacts. An alternative is by measurement of the (tightening torque) instead. Numerous lab tests had been conducted and proven that the (contact resistance) various with the (tightening torque). The (contact resistance) decrease about a straight line initially but flattens out up to a certain limit i.e. no advantage/refrain from over-tightening.
Q3. "...that wires terminated in lugs tightened to the recommended torque can easily be pulled out. Any movement in the wire tends to loosen them...."
A3. This may happen due to:
a) over-sized cable lug or under-sized conductor.
b) wrong selection of the crimping (tool and die) size which is too big for the lug. A correct (tool and die) would make a good crimp that the conductors are compressed into a (solid). NO conductor shall be able to be loosened from the lug. A good tool is designed such that the crimp is unable to be released (until it is fully crimped). There are unfortunately many poor quality crimping tools on the market place.
c) the crimping has nothing to do with the bolt tightening torque.
Che Kuan Yau (Singapore)

 

[waross]

Hi Mr.Che
I don't usually crimp set screw type lugs.
Other than that;
You need a few more decades in the field.
You won't find everything in the books.


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

 

[waross]

I have encountered damage to all of these types.
Damage that would have been avoided with a little lubricant.

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

 

[badservo]

It could be something in those boots causing corrosion. Try a different boot material/brand, if you can't find something else to replace it, the Penetrox or No-Alox may be enough to stop the corrosion.
To add to itsmoked's list:
5) Bolt too long, not fully threaded, or washers missing, which provides little or no clamping force.

 

[che12345]

Dear Mr. waross

W1. . " ...I have encountered damage to all of these types..."
C1. With due respect, All (four) types of lugs shown are NOT likely the the type of lug that Mr. SolarPrestige is using. This is based on the photos shown in (Mr. SolarPrestige)'s post. You are comparing apple and orange!
W2. " ... You need a few more decades in the field..."
C2. I am a septuagenarian and with more than 50+ years of hands-on field experience.
W3. " ...You won't find everything in the books."
C3. Agreed. Please share with us your learned experience if you have any different opinions on the points raised in my (three) posts.
W4. "...on copper connections and as a thread lubricant on the bolts..".
C4. the conduction paste is NOT intended (to be used) as a " thread lubricant on the bolts" . It would result to over-tightening!
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 ".
Che Kuan Yau (Singapore)