Tripping mcb and rccb

[Ndagger]

Hi guys.

Just want some advice or thoughts from the pro's.

So I'm an engineer who works on espresso machines. Going back a few months ago now I got called out to a machine tripping the power, in my line of work this generally means one loop on the heating element has shorted to ground or one of the 240v solenoid coils has got wet.
I got to site and no fault was found, trips stayed up and machine functioned ok. Nothing visual to be found so customer waved us away and said he'd call back if anything happened again.
2 weeks later same thing, mcb and rccb both tripped so I took a replacement machine and swapped his out assuming intermittent fault.

Wired his machine into our workshop circuit and tested everything, kept it on test for 2 weeks and put through it's paces, nothing happened that shouldn't have. Earth leakage on machine tested to be 0.018mA with element and pump running, coils all tested >999megohm with insulation tester. The machine we installed on his site never tripped his board either. After a few weeks on test he agreed to have his machine back and see what happens.

Machine was installed and fine for 1 month and now has tripped it's mcb and the rccb again so yet again I've collected and brought back to the workshop.
Customer is reluctant to have his wiring checked by an electrician based on the fact the machine we loaned him never tripped so it must be a fault on his machine.
I am seriously at a loss of what is going on and wondering if any of you pro's have suggestions.

I clamped the machine and at peak it pulled 26a. He has it on a 32A type B mcb on a split board with rccb

Any suggestions guys?

 

[ashrow]

If the circuit for the coffee machine is supplied by a rccb that protects more then 1 circuit an accumulation of small faults that add up can cause the rccb to trip

There are various tests an electrician can do on the installation rather then the machine.

If your machine runs/works ok on it own rcbo and the makers recommend it then the client needs to fit as per spec

I am assuming you have a 30ma rcbo on the supply of the appliance as you test it?

 

[Ndagger]

Thanks for your quick reply.

Yes, in the workshop I'm running it from the exact same set up he has on site, Schneider easy9 c32mcb and easy9 30ma rccb and no trips have happened.

I did suggest it may be an accumulation of leakage on the rest of his appliances but he's dismissed this suggestion and said that if that were the case the coffee machine mcb wouldn't drop too, only the rccb. I see where he's coming from on that but I am really struggling to find anything wrong with his machine and I've asked all the lads in our place to use it to death to try and get it to trip. Nearly a month of daily use and nothing even remotely interesting has happened

 

[Deleted member 105166]

in your OP you mention a type B breaker, then in your last post a type C. Please clarify.

 

[Lucien Nunes]

Intermittent faults can be like this. I have pieces of kit that I have been trying to recreate a fault on for years, that only malfunction when I'm hundreds of miles away. There can be all sorts of factors, from the installation position (e.g. more weight on one foot causes cable to be pinched inside) through to operator method (e.g. leans on it at a particular moment), environment (e.g. more humid in his shop) etc.

Make no assumptions, take nothing for granted. Step back and ask yourself what is different about the way he uses it and the way your volunteer testers use it. With only a few trip events, nothing is statistically significant so you might be trying to find a pattern in what it actually just randomness. Your best weapon is careful, systematic visual inspection. It's amazing how many gross faults an insulation tester can miss, if there's a smoke paper's clearance when you're testing.

 

[Pete999]

Thanks for your quick reply.

Yes, in the workshop I'm running it from the exact same set up he has on site, Schneider easy9 c32mcb and easy9 30ma rccb and no trips have happened.

I did suggest it may be an accumulation of leakage on the rest of his appliances but he's dismissed this suggestion and said that if that were the case the coffee machine mcb wouldn't drop too, only the rccb. I see where he's coming from on that but I am really struggling to find anything wrong with his machine and I've asked all the lads in our place to use it to death to try and get it to trip. Nearly a month of daily use and nothing even remotely interesting has happened

Has the actual circuit on site been tested? might be a circuit fault rather than a duff machinr

 

[littlespark]

He’s reluctant to have an electrician test his installation?
He should be having it done anyway to ensure safety for his staff and customers.

I think the replacement machine is done the same and he just doesn’t want to admit it.

Do you get the same 26A on both machines?

 

[Ndagger]

Tony Mitchell - sorry mate, late night typo. He has a B32 easy9 mcb on site, I use type C in the workshop of same brand.

The loan machine I installed pulls ~22A full whack, I did go down the route of it not pulling as much as his machine so I returned and swapped it for a second machine that draws 30A full whack, neither of my machines tripped his breakers.

The tripping was so random it didn't really coincide with anything in particular, sometime when pump was running with element off, sometimes sitting there doing nothing. His machine is an all singing all dancing model with touch screens and lots of electronics, when you switch on with the rocker switch it takes about 2 mins to "boot up". During this boot up procedure nothing is powered apart from the pcb and the screens, no element / pump / coils / cup warming element in circuit. The last time it went in his place he said it tripped the breakers as soon as he pressed the rocker switch, didn't even display the boot screen.

And yes I agree littlespark, he seems very reluctant to blame his install and I don't know why. I would have thought after all this time and use we've done with it, if there was something that caused enough overcurrent to trip a 32A breaker, we'd have seen it happen or at least seen or smelt what could be at fault by now?

 

[GBDamo]

Have you changed his breaker mayde up it to a C type if the circuit will allow.

Could be an oversensitive MCB that the 22A peak loan machine doesnt challenge but the 26A peak machine does.

The fixed wiring needs looking at, is it a single point circuit? there could be a poor connection in the switch or at the board.

 

[shaun1]

Is there a PSU in there to drive the electronics and touch screen etc. There could be a fault in there if it tripped on power up. Perhaps, as mentioned earlier, it is very humid in the shop and moisture is getting into the SMPSU.

It does, however, seem like a sensible next step would be to test the instalation. Not sure what your responsibility is in terms of repairing the machine but could it be worth you offereing to employ an electrician to do some basic tests on the instalation. It could save you a lot of time and money in the long run if the fault does turn out to be on the instalation rather than the machine.

 

[Ndagger]

Thanks for all the suggestions folks.

Yes the pcb has a transformer on it which powers all of the low voltage electronics. It outputs 24v dc and from what I've seen on a scope it has a pretty nice stable output, even with all screens and leds on. I did start to suspect the pcb (or some component on it) but with the majority of it being quite small surface mount components I'm assuming if something has gone so wrong that it's passing enough current to trip a 32A mcb it would have let the magic smoke out and stained the board or at least be getting hot enough to evaporate some IPA that I've sprayed on the board?

I'm not afraid to admit it's a bit over my head at this point, as I say I've done every test I can think of on the machine and can't fault it. Admittedly I can't recreate the humidity of his shop but to be fair, my workshop has 2 huge coffee roasters in it with gas burners fired all day and large roller shutters that open and close all day long letting in this lovely British weather we've got so I can't be that far away from his environment to make that much of a difference.

Technically our involvement starts and ends with the machine so I'm not obligated to do anything with his installation, it's just boggled me and I'd love to get to the bottom of it as much as the customer would, I'm just at the end of what I can do and think we need to get the installation inspected to fill in the blanks.
I think the sticking points for the customer are "if it was the site it would have always done it and not just started happening" and "if it was the site it would have tripped with the 2 machines you loaned me and it hasn't"
I'm finding it hard to argue those points not being as clued up as you guys are.

So a couple of questions for me to you qualified sparks ....

What could happen on his site to make this start happening now? Trip go weak, someone added something into his coffee machine circuit or tapping into the neutral for it?

Would a build up of leakage take out the mcb for his coffee machine aswell as the rccb for any reason?

Does the fact the rccb go down everytime the mcb goes down say anything? If it was a dead short wouldn't just the mcb go down?

In an ideal world, should a machine such as a coffee machine pulling 26A really be on a dedicated circuit with rcbo protection rather than an mcb in the consumer unit?

Sorry for all the questions guys, I don't just want to hand him the machine back and say "not my problem" I'd really like to find out exactly what's going on with this one!

 

[UKMeterman]

I see a couple of answers:
1. A faulty customer RCD.
2. The coffe machine will have a small leak to earth and it could be taking the RCD over the limit if there are other appliances on the came circuit.

 

[shaun1]

It is quite odd for both the RCD and MCB to both trip at the same time. A large fault to earth could cause both to trip, but the RCD would normally operate first in around 25mS. For a B32 to trip in under 400mS (and take out the magnetic trip) you would need a current of around 160A, which I would think would leave some evidence behind. And even then it seems unlikely that they would both go together.

I guess it could be a faulty MCB, which is taking the RCD out with it.

Are you sure both devices always trip together. Is the customer definitely giving you the full story?

 

[Lucien Nunes]

To me, the fact that two separate protective devices trip at once is a good indication that there really is a fault, an intermittent but solid short from line to earth. The MCB will tend to trip because of the high fault current, and the RCD will tend to trip as that current is an earth leakage. The two will race each other to disconnect the circuit and as Shaun says unless the current is very high the RCD might well go first, but they can both trip from the same fault. Excessive total leakage wouldn't trip the MCB, borderline overload wouldn't trip the RCD.

If the coffee machine circuit has been tapped into for something else, that something else might be the fault, but it is really just as likely to be the circuit cable itself shorting where it has been chewed by a mouse, trapped under a floorboard, hit by a pallet truck or whatever. We've seen cables with a screw through that lasted for ages in continuous use, then one day a tiny amount of thermal movement caused the screw to touch two conductors at once for the first time. A dead short will cause a splash, remove a bit of copper and leave a tiny gap once more, until something moves again.

There's no specific reason for the circuit to be on it's own RCBO unless high leakage was expected. That might be how you would do it now, but RCBOs are relatively new; the majority of extant electrical installations were put in before they were available.

You've tested the insulation of the machine, now the logical thing to do is to test the insulation of the system. I know that isn't what you want to say to the customer but it really is the next sensible step. It may well come back clear, and frustratingly, that doesn't prove it's not a wiring fault, just that the fault has blown itself well clear for now.

 

[Ndagger]

Lucien Nunes & shaun1, awesome feedback thanks. I've never seen the trip in person so only have the customer info to go on but each and every time he says the mcb and rccb have both gone. The last time it went he told me after it tripped he knocked down all mcb's except coffee machine, put rccb back up and it blew straight away, he then turned machine off at its rocker switch and reset the coffee machine mcb and the rccb and both held up, as soon as he switched coffee machine rocker switch on the rccb and coffee machine mcb both dropped instantly.

As I say, I've not seen it trip in person but the guy seems straight up so I guess I take his word on it all.
Might have to bite the bullet and pay a spark to go and inspect since he doesn't seem willing too