Busbar & Tap off box - power loss & Over-voltage issues

MarkyofftheMark · Oct 9, 2018

On one of my sites, I have a 6 rising busbars ( E & I powerbar) rated at 250 A connecting a tap off box with a Schneider 100A isolator feeding a 3-phase board with 16mm SWA. I have around 24 tap off boxes onsite across 6 busbars and the installation is approximately 6 years old.
Over the past year I've had instances where the power has been completely lost. On shutdown and Inspection the busbar has burn marks and this has dropped out the phase; I've had to file the busbar and the tap off box connector to clear the residue in order to get this working.
This has happened to another box and the above works were carried out, but since we have had the voltage acting erratically and have been picking up 350V on two of the phases with just 50v on the third, which has burned out a significant amount of voltage sensitive equipment. The load on the board is well within the current carrying capacity and the SWA connections are secure on the tap off boxes. I've enclosed some pics to demonstrate.

[ElectriciansForums.net] Busbar & Tap off box - power loss & Over-voltage issues

Has anyone else come across this or had experience of this.

davesparks · Oct 9, 2018

I would hazard a guess that the damaged connection is on the neutral. The loss of the neutral to a three phase board will cause this rise in the voltage on some phases.

The busbar tap off unit will have been damaged by the overheating and the spring contacts will have lost their springiness.
By filing the busbar and the contacts you will have removed metal making the connection worse and causing this complete loss of the neutral.

MarkyofftheMark · Oct 9, 2018

Thanks for this. I've checked the neutral on the 3 phase board and it seems in order.

davesparks · Oct 9, 2018

MarkyofftheMark said:
Thanks for this. I've checked the neutral on the 3 phase board and it seems in order.

But it’s the neutral in the tap-off and busbar which is damaged and causing the issue. This is obvious from the pictures you have posted.

MarkyofftheMark · Oct 9, 2018

the neutral in the Tap off box is secure on both sides of the isolator and there are no signs of damage to the cable. Could it be the neutral at the origin of the busbar is the issue, and if this is the case, why is only this tap off and one more affected on another run?

Pete999 · Oct 9, 2018

MarkyofftheMark said:
the neutral in the Tap off box is secure on both sides of the isolator and there are no signs of damage to the cable. Could it be the neutral at the origin of the busbar is the issue, and if this is the case, why is only this tap off and one more affected on another run?

Maybe this tap off is the one with the most load on it? without eyes on it's very difficult to form a proper opinion.

MarkyofftheMark · Oct 9, 2018

Absolutely, really appreciate your comments. The load on this board is no greater than any of the other 20+ boards. Has me slightly baffled. My thoughts are the tap off box connection wasn't great and its arced over time and caused the problem I'm experiencing now.

Pete999 · Oct 9, 2018

MarkyofftheMark said:
Absolutely, really appreciate your comments. The load on this board is no greater than any of the other 20+ boards. Has me slightly baffled. My thoughts are the tap off box connection wasn't great and its arced over time and caused the problem I'm experiencing now.

Are the blades showing signs of slackness? Does the DB this particular tap off feeds, have predominately SP&N circuits, or is it about the same as the rest of the install? Is the DB reasonably well balanced across all three phases?

MarkyofftheMark · Oct 9, 2018

The Tap off box fitted very securely to the busbar and doesn't jiggle or move when connected. The majority of circuits are SP & N with a couple of three phase MCB's.

Pete999 · Oct 9, 2018

MarkyofftheMark said:
The Tap off box fitted very securely to the busbar and doesn't jiggle or move when connected. The majority of circuits are SP & N with a couple of three phase MCB's.

Have you checked the current draw on the 2 phases, are there any big differences?

Wilko · Oct 9, 2018

My 20p worth - once arcing has occurred in that tap off perhaps it should be replaced rather than repaired. Also, perhaps check the othe tap offs in the site (that are ok) are the same type.

MarkyofftheMark · Oct 9, 2018

I have monitored the current via the reader on the Schneider board and the current draw is at its peak around 25-30A which is consistent. I've had to isolate the whole board as the lights were flashing and chargers plugged into sockets were smoking.

MarkyofftheMark · Oct 9, 2018

Wilko said:
My 20p worth - once arcing has occurred in that tap off perhaps it should be replaced rather than repaired. Also, perhaps check the othe tap offs in the site (that are ok) are the same type.

yes, tap off boxes already on order.

Pete999 · Oct 9, 2018

MarkyofftheMark said:
I have monitored the current via the reader on the Schneider board and the current draw is at its peak around 25-30A which is consistent. I've had to isolate the whole board as the lights were flashing and chargers plugged into sockets were smoking.

As I said no eyes on, clutching at straws now, other than loose connections, who knows?

Wilko · Oct 9, 2018

MarkyofftheMark said:
...I've had to isolate the whole board as the lights were flashing and chargers plugged into sockets were smoking.

Hi - As @davesparks has said - the over voltage you are seeing must be from lost N. That has to be found and fixed.

marconi · Oct 9, 2018

What is odd is the way the busbars have been connected to the live conductors viz: Brown, Blue, Black, a gap then Grey. This is wrong.

One would expect Brown, Black, Grey, a gap, then Blue so that the electromagnetic fields of the three lines combine to reduce in magnitude the resultant field around them. The Grey out on a limb to the right markedly stops this happening.

In the correct order Brown, Black, Grey a gap and then Blue, the effective situation is two parallel conductors ( one made up by the 3 lines and the other the neutral), with the current flowing in them being in opposite directions so the net em field produced by each current is minimised/cancelled out. This cannot happen as well in the actual set up shown in the pictures because the Grey is separated.

So, as well as the advice from others about dodgy connections the busbar ordering of live conductors exacerbates the electromagenetic effects of any transient currents. From Lenz's law rapidly changing strong EM fields around conductors induces high voltages. Perhaps this is the cause of the arcing.

You might want to investigate the nature of the loads on the failing tap off boxes and look for similarities between them not seen with the remaining tap off boxes.

marconi · Oct 9, 2018

You ought to check whether the busbars are actually correctly connected to the lines and neutral but it is the tap off box which is incorrectly wired to the busbars ie: Black, Grey and Blue (- if Brown is correctly terminated you would expect to see the Black on the second from left and then Grey which your image does not show ). The isolator might be switching the neutral feed which is not intended. You'd get some odd voltages if this was the case AND A DANGEROUS SITUATION.

Andy-1960 · Oct 9, 2018

It also looks as though those bus bars are tinned or plated to improve conductivity, filing the bars will have removed this.

marconi · Oct 9, 2018

marconi said:
You ought to check whether the busbars are actually correctly connected to the lines and neutral but it is the tap off box which is incorrectly wired to the busbars ie: Black, Grey and Blue (- if Brown is correctly terminated you would expect to see the Black on the second from left and then Grey which your image does not show ). The isolator might be switching the neutral feed which is not intended. You'd get some odd voltages if this was the case AND A DANGEROUS SITUATION.

IGNORE MY #17. I have got myself addled with the above post but cannot edit it because 20 minutes has passed. What I want to say is go back to fundamentals and confirm the energisation of the busbars with respect to the three supply lines and neutral and then check how the connections at the top of the isolator have been made to them.

davesparks · Oct 9, 2018

MarkyofftheMark said:
the neutral in the Tap off box is secure on both sides of the isolator and there are no signs of damage to the cable. Could it be the neutral at the origin of the busbar is the issue, and if this is the case, why is only this tap off and one more affected on another run?

It’s not the connections inside the isolator that I am talking about, it is the connection to the busbar itself. You can see burning on the busbar itself and not inside the isolator, therefore the problem is at the point at which is connects to the busbar.

The tap off will need to be replaced and moved to an undamaged part of the busbar. Fitting a new one in the same location will result in it burning out again due to the damaged busbar.

Simonslimline · Oct 9, 2018

marconi said:
You ought to check whether the busbars are actually correctly connected to the lines and neutral but it is the tap off box which is incorrectly wired to the busbars ie: Black, Grey and Blue (- if Brown is correctly terminated you would expect to see the Black on the second from left and then Grey which your image does not show ). The isolator might be switching the neutral feed which is not intended. You'd get some odd voltages if this was the case AND A DANGEROUS SITUATION.

The last job i was on had some crappy legrand busbar system installed throughout the multi story carpark and main building. The connections were, L1-N-L2-E-L3 Where the SWA,s terminated in to the feeder box for the busbar.

I have looked online but cant find the same busbar.

Fozzy0207 · Oct 12, 2019

Simonslimline said:
The last job i was on had some crappy legrand busbar system installed throughout the multi story carpark and main building. The connections were, L1-N-L2-E-L3 Where the SWA,s terminated in to the feeder box for the busbar.

I have looked online but cant find the same busbar.

This is the standard confirmation for busbar I have worked on. Mainly Eaton and E&I. The pitting on the bar will be as a result of a poor contact between the tap off fingers and the bar itself. Depending on the damage you will need to move the tap off position and potentially change out the length of bar as well. The fingers on the tap off are probably slack and will need replacing. A quality issue and may be covered under warranty. If you can’t move the tap off position I would change the fingers and depending on the damage clean up the bar and reinstate making sure to use good contact grease.

Lucien Nunes · Oct 14, 2019

Is this tinned aluminium busbar by any chance? (MPB is available with either copper or ali bars). If so, once damaged, the aluminium is exposed and will not make proper contact with the tap fingers even if it is cleaned up and tightened securely. Aluminium forms an insulating oxide layer instantly on contact with air, which requires abrasion under a layer of anti-corrosive paste to eliminate. A tap clamped on to bare ali could certainly make such poor contact that arcing would begin at low currents, or just be high resistance without arcing.

I don't have the MI's to know the official termination procedure but as above, I would start with a new set of fingers in a new position and make sure to proceed carefully regarding maintaining plating integrity.

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