RCCB Tripping On Ship

[oceanview]

I am a marine engineer working onboard a smaller vessel struggling with RCCBs tripping on two of the distribution boxes. Electrical equipment is not my specialty so I am hoping for some guidance and help.
We are currently connected to a shore power supply of 380 Volts, 50 hz. This is fed through an isolation transformer and then to the main switchboard. From here through 3 pole breakers several distribution boards are supplied. Each distribution board is connected to the neutral bar on the switchboard as well as the ground bar on the switchboard.
On two of the distribution boxes the RCCBs trip immediately when any load is run through the RCCB. In other words, if I open all of the breakers supplied by the RCCB, it will latch closed. As soon as I close any of the breakers, whether it be lights, an air handler, or a stove, the RCCB trips. I can close a socket breaker and the RCCB will stay closed until I plug something into the socket (such as a light) and the RCCB immediately trips. Both of the boards began doing this at the same time.
I have tried disconnecting the neutral cable in the swithcboard to the RCCB and megger testing to ground to see if it has been damaged, but it is fine.
I have shut off everything on the entire vessel and only supplied power to the distribution board and the same thing happens.
Three other boards fitted with RCCBs are functioning fine and take the supply from the same bus bars in the switchboard and their neutrals are also on the same bar.

Any ideas?

I read from a previous post that RCCBs should only detect faults further down stream. Which means the problem should be in the distribution boards themselves. Normally, the typical process for troubleshooting something like this is to switch off all of the breakers on the distribution board and then switch them on one by one to find which one is the source of the problem. But all of them trip...

 

[dpc]

It sounds like there is a wiring problem between the neutral and ground.

The RCCB trips when it detects a difference in current between the phase ("hot") conductor and the neutral conductor, because this indicates some of the current flowing in the hot wire is not returning on the neutral conductor, meaning some current is flowing where it shouldn't be.

For this system to work, the neutral conductor must be insulated from ground and there must be no neutral-ground connections downstream of the RCCB.

You should take a close look at the neutral and ground buses in the panel. It sounds like the incoming neutral may be connected to the ground bar, or the panel has a bonding jumper between neutral and ground that needs to be removed.

 

[itsmoked]

Got some more history?

Has this all been working fine at other ports?

Is this a new start up?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[oceanview]

itsmoked: We are currently busy with over a year long refit. The distribution boxes have had some work done on them, limited to replacing ceramic fuses with breakers. However, I have been running both of the distribution boards which I have had problems with for over a week without any problems until a couple of days ago when I came into work and both RCCBs were tripped.

dpc: I will have closer look at the buses on the distribution boxes tomorrow once I am back at work.

What is really strange for me is that two seperate distribution boxes have this problem, but they have independent cables running from the switchboard including individual neutral supplies (they do end up on the same neutral bar in the main switchboard). This should allow me to completely isolate one board from the other and you would think that a problem with one board would not affect the other one.

 

[itsmoked]

Yeah a tough one.

Keep in mind that a neutral to ground error can be anywhere on any of the loads serviced by one of those RCCBs because your various load breakers don't disconnect the neutrals and grounds.

This means the problem doesn't have to be on the actual panel. The result would be the same. No loads at all works, but as soon as a load is applied the RCCB detects any ground/neutral short.

Since you are in the midst of a retrofit was an appliance or device installed in two places that just happen to be on the different RCCB circuits?

Beware that they could actually have put in 20 items scattered on both your RCCBs and any one of them would cause your issue.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Turn everything off and measure the resistance between the neutral and the ground on those two RCCBs.

My concern is that, say, 20 devices were installed that all tie neutral to their cases.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

It sounds as if your system has a grounded neutral. If so, your neutral should be originated at the isolating transformer with no direct connection to shore power from either the ship neutral or ship ground. Connecting a grounded system neutral to a shore based neutral may result in severe hull corrosion.
BE AWARE that hull currents and attendant corrosion may be caused by another ship on shore power, and the ship causing the issues may not itself have hull current issues.
Another possible source of problems is electric welding machines supplied from shore power.


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

 

[oceanview]

Thanks for all of the tips, found out that the neutral on one disconnected circuit was ground. I found that my mistake was when I started troubleshooting the problem was that I assumed if all of the circuits on one of the distribution boards were off, I would be able to find the fault by switching the breakers on one by one in order to find the faulty circuit once the RCCB tripped. However, since all of the neutrals from the individual circuits run to a common neutral, as soon as I put power on one any one of the circuits, the neutral would be connected to the faulty grounded neutral throuhg the neutral bus. I ended up taking all of the neutral connects on the bus off one by one until I found the grounded one.

 

[itsmoked]

Attaboy! Lotta work but it cut to the chase.

So it was just a loose wire; flying neutral?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rovineye]

I have seen few of these type distribution systems on ships, but I do have one. I have individual breakers with GFCI type breakers with another monitor on the neutral lead between the neutral bus and transformer wye connection. Based on your description, I would have seen the neutral bus monitor trip and not an individual GFCI, or maybe one GFCI depending on where the errant connection was.
In your case you couldn't trace the fault like we do for ground faults by securing breakers one by one. It would have fooled me for a while as well.

 

[oceanview]

Itsmoked: The faulty circuit on one board was the carpenters damaging some cables and the second circuit I am not sure yet. Unfortunately the second board does not have the neutrals labelled, so I am not sure exactly which one is grounded. My plan with this one is that once most of the work in the area is done and I can start testing all of the circuits on that board I should be able to find out which breaker either trips the rccb or does not work since I have the neutral disconnected.

One final thought on this subject, wouldn´t it make more sense to have two pole breakers for each circuit (one pole hot and the second pole connecting the circuit to the neutral bar) so that if there was a problem with a grounded neutral then you could actually go through the breakers one by one and the RCCB would trip once the faulty circuit was energized? Other than the fact that the size of the distribution board would have to be twice are there any drawbacks to this type of system?

 

[itsmoked]

Only real drawback is a lack of breakers made for that. And any multi-pole breakers are made for connecting to multiple phases. Availability.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[rovineye]

Yes, you can switch the neutral.
IEEE Std 45 Recommended Practice for Electrical Installations on Shipboard (2002) Sub Clause 17.1 Distribution Panels "Circuit breakers in grounded neutral distribution panels may include a pole to simultaneously switch the neutral."
Cost and size are the drawbacks as you said.