RCBO failure in an unbalanced split phase installation

[Ohmer Simpson]

Hello all, this is my first post on any forum so please forgive any ineptitudes. I'm a relatively recently qualified electrician and have encountered a single phase /split phase 3 wire installation for the first time. This installation is fed through a three phase board containing mccb's, however prior to this the red and yellow phases are taken from the same buzz bar. Obviously one would think loading would be considered when making the transition from split phase to distribution of two lines upon a 3-phase board. However this is not so. On all 12 boards a measured voltage of 15v is apparent and the same is true of all outlets, clearly the neutral is carrying significant current. Which brings me to my question I've just been testing a board full of RCBO's. When measuring earth loop impedance on a no trip setting (using a megger mft 1730) not only are the RCBO's tripping they are mechanically failing, burning out and smoking. Obviously I'm putting this down to the imbalance in the system but can't understand the science that explains why, could anybody please explain this for me

 

[Knobhead]

Take a N to E voltage readings.
I’m sorry but your description of the fault is hard to follow. I’m going for a faulty neutral connection, not necessarily in your installation but possibly the supply company.

 

[Engineer54]

Take a N to E voltage readings.
I’m sorry but your description of the fault is hard to follow. I’m going for a faulty neutral connection, not necessarily in your installation but possibly the supply company.

I'm glad someone else couldn't make any sense of this discription. I read it several times and was more confused each time i read it. ..lol!!

 

[HandySparks]

Hello all, this is my first post on any forum so please forgive any ineptitudes.

Welcome to the forum. Now a few questions in order to understand the problem:

I'm a relatively recently qualified electrician and have encountered a single phase /split phase 3 wire installation for the first time. This installation is fed through a three phase board containing mccb's, however prior to this the red and yellow phases are taken from the same buzz bar. Obviously one would think loading would be considered when making the transition from split phase to distribution of two lines upon a 3-phase board. However this is not so.

Just for background, can you say what type of premises this is in?
A diagram of the supply would be good. Or some photos. Or just a step by step description from the incoming supply cable to the final circuits.

On all 12 boards...

What 12 boards?

...a measured voltage of 15v is apparent and the same is true of all outlets, clearly the neutral is carrying significant current.

15V measured between which two points?

Which brings me to my question I've just been testing a board full of RCBO's. When measuring earth loop impedance on a no trip setting (using a megger mft 1730) not only are the RCBO's tripping they are mechanically failing, burning out and smoking. Obviously I'm putting this down to the imbalance in the system but can't understand the science that explains why, could anybody please explain this for me

How are the RCBOs "mechanically failing"? What do you see? Is this happening as a result of your ELI testing?

Lots of questions but necessary for an attempt at an answer.

 

[Ohmer Simpson]

Thank you for taking the time to read this and for responding to (on re-reading) is an under explained situation. However I think today I've worked out the issue with the RCBO's, but would appreciate it if any of you could point out any failings or inaccuracies I'm stating. As you have all said I didn't lay out the circumstances very clearly. My measured voltage is between neutral and earth. The Ze is 0.34. I have a split phase system spread across a 3-phase board distributing around an hospice (Built 1902). Red and yellow phase have been linked out at source, so if theoritically the 2 lines were spread evenly across a 3-phase board the loadings would proportioned 66.6A L1 -33.3A L2 of a 100A supply. I'm finding 15v between neutral and earth so working to the worst case scenario of 0.34ohms I reckon there is 45A on the nuetral. Therefore my suggestion is.... there is current back feeding from the neutal bar to the RCBO. However because this is incapable of energising the coil if the switch is closed it is going to remain closed as the coil sees no imbalance, however once it is opened (I have found today that just by manually opening the breaker, it will not close again), the current backfeeding through the rcbo is such that the bi-metalic strip is put under to much current to retain it's working charecteristics. Does this (a) make sense and (b) Appear plausible? once again thank you for your time

 

[i=p/u]

interesting, and hope someone can picture you situation... sorry not me

 

[HandySparks]

I'm still lost. A diagram would really help. Can you measure this suggested 45A with a clamp meter?

 

[Knobhead]

Ohmer were are you in the world? I’m sorry but I can’t make any sense of your posts. You need a local electrician to help you.

 

[Mark42]

... I have a split phase system spread across a 3-phase board distributing around an hospice (Built 1902) ...

Eh?

Is the incomer two or three phases? Since you call this 'split phase' I guess only two? But maybe using standard three-phase boards with two phase conductors linked?

What's the voltage between the phases? That will reveal at least the type of supply.

Otherwise I'm finding it impossible to understand what you're on about! Sorry

 

[wodanbrlam]

More good ideas,thanks guys, yep il post some picks as soon as I have a few items ready