Discuss Liklihood of adjacent RCBO tripping due to fault on another circuit? in the Electrical Forum area at ElectriciansForums.net

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Got an odd one.

Supply is TPN TN-CS and all looks reasonably tidy but the circuit schedules bear little resemblance to the DB contents.

I have a piece of fixed equipment that is generating intermittent faults and tripping the C16 RCBO. The circuit tests fine at 999MOhms at 500V on IR. So it's looking like the fixed equipment or it's associated wiring.

The curious thing is that they are claiming that when it trips it takes down the adjacent B32 RCBO.

I have ramp and RCD tested the C16 20 times and not got the B32 to trip.

The wiring in the board is untidy but all is secure and correctly wired.

My theory is that when restting the faulty C16 they are hitting the test button of the B32.

But thought I'd ask if there were any exotic scenarios where one circuit fault can cause another to trip?
 
a N-E fault on a circuit can cause an unrelated RCD to trip when a load is switched on on the non-RCD (or a second RCD) side.
Thanks but, to clarify.

I have a full RCBO board.

Are you suggesting that if my fixed equipment circuit, the C16 had a N-E fault it could cause another RCBO to trip when put under load?

As all the IRs test fine this is unlikely, yes?

Or...

That when another circuit has, when under load, a N-E fault that may be causing the C16 to trip?

At the moment I'm going for fat fingers.
 
had a N-E fault on a shower once. the shower was on RCD1. as soon as a load (kettle) was switched on on RCD2, RCD1 tripped. something to do with the N current finding it's way through parallel paths like bonding through the tripping RCD. can't explian the theory without burning my few brain cells out.
 
Given that magnetic fields don't respect a plastic housing, I guess it's not impossible that if you had a large enough spike in one device that it might cause some disruption to an adjacent coil, possibly encouraged if there were some harmonics involved as well. A simple test would be to swap them around on the bar and see what aspect the fault decides to follow.
 
Just been testing the ZDB and got 0.06 L-E and 0.05 L-N so no issues with the supply.

Id like to ramp test the B32 to see if it is failing early but that circuit has the server on and shuts down the site.

Think I get my super precise Tacklife earth leakage clamp meter out and have a root round.

The B32 may have a high baseline leakage???
 
Given that magnetic fields don't respect a plastic housing, I guess it's not impossible that if you had a large enough spike in one device that it might cause some disruption to an adjacent coil, possibly encouraged if there were some harmonics involved as well. A simple test would be to swap them around on the bar and see what aspect the fault decides to follow.
Like that, problem is I can't isolate without shutting the site down.

Will likely be returning out of hours so would like to have potential solutions ready along with parts.
 
Just been testing the ZDB and got 0.06 L-E and 0.05 L-N so no issues with the supply.

Id like to ramp test the B32 to see if it is failing early but that circuit has the server on and shuts down the site.

Think I get my super precise Tacklife earth leakage clamp meter out and have a root round.

The B32 may have a high baseline leakage???
If that's a circuit supplying solely IT then I'd be tempted to put a B type RCD at 100mA anyway, but depends on how it's all wired - socket outlets etc (and also if you can get a B32 B100 RCBO for that board!!)
 
Unf
If that's a circuit supplying solely IT then I'd be tempted to put a B type RCD at 100mA anyway, but depends on how it's all wired - socket outlets etc
Unfortunately not, it does a staff kitchen as well.

Need really to get that circuit tested just can't at the moment.
 
Are you suggesting that if my fixed equipment circuit, the C16 had a N-E fault it could cause another RCBO to trip when put under load?

No, the N-E fault would be on the B32 circuit. With N-E fault trips, the load / overload / fault that triggers the trip event can be on any circuit, but the pre-existing N-E fault that will be on the RCD / RCBO that trips unexpectedly. Rather than a solid fault, it can also be borderline operational leakage plus the effect of the supply disturbance.
 
If the fault is a big one, then it is possible both for magnetic coupling to be a culprit (as mentioned above) or even the dV/dt on the supply as it clears huge currents resulting in enough capacitive flow to trip a RCD/RCBO.

Certainly it sounds like a hard fault as IR results are reported to be good. Though I guess it is theoretically possible for vibration on tripping to do it, but you could rule that out by tripping the 'faulty' circuit using your MFT and see if a low-current fault ever does it.
 
Had something similar on a dual RCD board, testing one RCD and the other tripped as well, what was more confusing I inavertantly did an RCD test on the outgoing terminals of the main isolator and RCD No2 tripped again, I eventually located some minor cable damage on one of the outgoing circuits that appeared to be causing the problem
 
What is the item of fixed equipment supplied by the 16A RCBO? Is it single phase? Is it on the same phase as the 32A RCBO final circuit?

And to confirm - the staff kitchen sockets are on the same supply to the important IT server. Mmmm - not wise methinks.

The DB is TPN or SPN?

Do the RCBOs have a functional earth?

Are the RCBOs single or double pole?
 
What is the item of fixed equipment supplied by the 16A RCBO? Is it single phase? Is it on the same phase as the 32A RCBO final circuit?

And to confirm - the staff kitchen sockets are on the same supply to the important IT server. Mmmm - not wise methinks.

The DB is TPN or SPN?

Do the RCBOs have a functional earth?

Are the RCBOs single or double pole?

The item is a 70KV intraoral X-ray. It is single phase and on the same phase as the B32.

The DBs, 3 of, are all single phase, one on each.

Yes, functional earth.

Not sure on the number of poles of the RCBOs, could Google it and pretend I know what I'm doing but can't be arsed.

For a company with 720 sites across the UK and Ireland their attitude to electrics Is deplorable.
 
One of the directors of that company is responsible for the Health and Safety of the workforce and assets, maybe a small nudge is needed to remind them of their responsibility or worse the danger they are putting their operatives in, start at the facilities manager, they will not want to take the responsibility and soon refer it upwards, but then maybe they too can't be a***d.
 

Some key remarks on 'ampacity' at 1min20sec.

I wonder if the tripping has only just begun to happen which would indicate some sort of failure leading to increased earth leakage or current drawn. What follows assumes it has only begun to trip.

I suspect it is intermittent tripping and happens depending on when the X ray valve is pulsed on in the cycle of the mains voltage waveform. The switching on and off of the X ray valve will take place on the mains side of the transformers. Typically the X ray valve current ay 70kV is 7mA - I will use 10mA for my estimates. These are peak values.

70 000 x 0.01 = 700Wpk. Not a lot for a 16A mcb/RCBO but the current will not be sinusoidal but more like attachments 1 and 2 depending on the nature of the LV to HV power supply being inductive or capacitive.

The RCBO's fast magnetic overcurrent trip may be tripping because of a very high peak current if the X ray is pulsed on when the sine wave is crossing the time axis for an inductive transformer or at peak voltage for a capacitive switched mode PSU.

Why would this affect the 32A RCBO. The high but brief current of the X ray machine may:

a. create a voltage drop along the neutral feeder to the DB sufficient for the RCBO's functional earth electronics to 'think' the neutral has become open circuit;

b. the 32A RCBO's electronics is sensitive to the voltage disturbance;

c. the EMC/EMI filters in the IT server respond to the short but fast voltage disturbance when the X ray pulses on and off by creating earth leakage current L-E and N-E the sum of which is not zero and greater than the prevailing trip threshold bearing in mind the IT server is on a final circuit with other kitchen equipment.

What to do? Some suggestions on what I would try:

1. Establish if the 16A RCBO is the correct rating and type for the X Ray machine;
2. If the wiring allows, swap 16A RCBO for a 20 or 30A RCBO and see if further trips happen again considering type of RCBO;
3. Temporarily and only during you attendance on site connect the DB's N and E busbars together using at least 6mm2 - this ties the N and E voltages together irrespective of the surge current through the feeder N conductor;
4. Move the X ray supply to another phase - may still need to uprate the RCBO though.
5. Upsize the feeder to the DB.

:cool:
 

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Some key remarks on 'ampacity' at 1min20sec.

I wonder if the tripping has only just begun to happen which would indicate some sort of failure leading to increased earth leakage or current drawn. What follows assumes it has only begun to trip.

I suspect it is intermittent tripping and happens depending on when the X ray valve is pulsed on in the cycle of the mains voltage waveform. The switching on and off of the X ray valve will take place on the mains side of the transformers. Typically the X ray valve current ay 70kV is 7mA - I will use 10mA for my estimates. These are peak values.

70 000 x 0.01 = 700Wpk. Not a lot for a 16A mcb/RCBO but the current will not be sinusoidal but more like attachments 1 and 2 depending on the nature of the LV to HV power supply being inductive or capacitive.

The RCBO's fast magnetic overcurrent trip may be tripping because of a very high peak current if the X ray is pulsed on when the sine wave is crossing the time axis for an inductive transformer or at peak voltage for a capacitive switched mode PSU.

Why would this affect the 32A RCBO. The high but brief current of the X ray machine may:

a. create a voltage drop along the neutral feeder to the DB sufficient for the RCBO's functional earth electronics to 'think' the neutral has become open circuit;

b. the 32A RCBO's electronics is sensitive to the voltage disturbance;

c. the EMC/EMI filters in the IT server respond to the short but fast voltage disturbance when the X ray pulses on and off by creating earth leakage current L-E and N-E the sum of which is not zero and greater than the prevailing trip threshold bearing in mind the IT server is on a final circuit with other kitchen equipment.

What to do? Some suggestions on what I would try:

1. Establish if the 16A RCBO is the correct rating and type for the X Ray machine;
2. If the wiring allows, swap 16A RCBO for a 20 or 30A RCBO and see if further trips happen again considering type of RCBO;
3. Temporarily and only during you attendance on site connect the DB's N and E busbars together using at least 6mm2 - this ties the N and E voltages together irrespective of the surge current through the feeder N conductor;
4. Move the X ray supply to another phase - may still need to uprate the RCBO though.
5. Upsize the feeder to the DB.

:cool:
Awesome response, many thanks.

A bit more info from yesterday.

We ended up calling a in an X-ray engineer who put the thing through a number of calibration runs, fully stressing the unit.

We managed to get it to trip once but not at a significant point, during a quick exposure, and not repeatable.

To be honest we're bit stumped but the engineer has put in a warranty claim for a new head, for the X-ray.

If this fails we'll run in a new supply, easier in this case that casing down a fault.

The tripping 32A is, almost certainly, a red herring. I managed to get a few of the staff together to show me them resetting the C16 and of the five, two activated the test button on the 32A whilst doing so.

None were aware of what the test button was and what it did.

These X-ray units, and most others, stipulate a D16 device however, of the hundreds I've seen, I've only ever seen one on a D16 and I had to change that to a C16 as the ZS of the circuit was too high for 0.4 second disconnection.

We have to be aware that most dental practices are domestic installations so its rarely possible to fit a D type.

In this particular practice all their X-rays, seven in total, are on B6 RCBOs.
 
Thanks but, to clarify.

I have a full RCBO board.

Are you suggesting that if my fixed equipment circuit, the C16 had a N-E fault it could cause another RCBO to trip when put under load?

As all the IRs test fine this is unlikely, yes?

Or...

That when another circuit has, when under load, a N-E fault that may be causing the C16 to trip?

At the moment I'm going for fat fingers.
I have the same issue when I replaced the switchboard with new RCBOs. I don't know the solutions. I can only have power to either one rcbo at a time. I cannot have both the rcbos on together else they will trip at the time together. Sometimes they trip instantly and other times after a few minutes. If I only power on one of either of these two rcbos at a time they will not trip.

One rcbo controls the lights upstairs and the other controls the powerpoints upstairs.
 
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I have the same issue when I replaced the switchboard with new RCBOs. I don't know the solutions. I can only have power to either one rcbo at a time. I cannot have both the rcbos on together else they will trip at the time together. Sometimes they trip instantly and other times after a few minutes. If I only power on one of either of these two rcbos at a time they will not trip.

One rcbo controls the lights upstairs and the other controls the powerpoints upstairs.
What testing have you done on both circuits sounds like both circuits have a fault that when they are combined they trip the RCBO's in question
 

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