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?

 

[Lucien Nunes]

I was thinking about replacing the parts at your cost, not the customer's, but it seems that would be a bit expensive unless you really want to keep him on side.

It takes a reasonable size triac to repeatedly knock out a B32 without itself turning from a semiconductor into simply a conductor. Even something like a BTA40 that would switch the total load of the machine only stands up to maybe 1000A²s of fuse let-through, e.g. an F16 HRC. I've no idea what triacs are fitted to the coffee machine but they shed a slightly different light on my 'probably an element,' unless you normally find that the triacs do survive an element short.

I am beginning to wonder if the customer's report was correct, that the MCB tripped when the switch was operated. It is so much less likely than just the RCD tripping, especially in the absence of any installation IR test results.

In the meantime, your testing is obviously not representative. Why not bring the machine along to our workshop. Show us how to use it, give us a good supply of coffee and we'll test it to the best of our electrical abilities. If we ever find the fault, we'll let you know.

 

[marconi]

Ndagger: Thank you for your post. When you add together the power rating of the group elements and the water tank element what figure do you arrive at? Actually I'd really like to know the power of the individual elements as well. Then what is the power of the cup warmer element and the kW/hp of the compressor motor?

Am I picking up correctly that in your workshop (fed by 32A type C)and then when worked hard in the training centre (fed by what size/type of mcb??) for baristas you have not experienced a problem with tripping? If so, then the setting of the machine in the clients premises may well be relevant hence my additional line of investigation about the mcb being derated by temp perhaps?

What does the manufacturer say about type and size of mcb for the machine?

Measure the mains voltage in your workshop and also at the client's next time you visit.

Would you know if the triacs are switched at zero-crossing or phase angle? And lastly, when you say triacs do you mean such devices in a module called a solid state switch/relay which is a little block with 4 terminals, two for load and two for control? If they are SS switches are they random or zero-crossing turn on? If you sent me details I can look them up.

I am wondering - in the absence of information - if the total installed heating power is greater than 6.4kW but triac based heater control technology is limiting the machine's consumption to circa 6.4kW. Maybe there is some spurious triggering of triacs causing an excessive over current great enough to trip the 32B mcb (already hot or de-rated or both?).

A quality of electricity supply problem at the clients causing the spurious triggering? - fast dV/dt of the line -neutral voltage can if not filtered cause triacs which are not gated on nevertheless to be turned on and in your case at the 'wrong time'.

 

[Ndagger]

Lucian nunes - coffee we've got, by the tonne! Not sure when I'll be passing through London next though but if I do I'll throw the machine in the van!

Marconi - don't have all the answers to hand now but if I get a chance tomorrow I'll get the panels off the machine and note down element wattages and motor rating. This machine doesn't use SSR's (some do though) this uses heatsink mounted triacs, will have to double check part numbers from the package but a lot of machines use tic253 (off top of my head) or similar. The coffee boiler elements are a lot smaller than the steam element, the steam element in most machines have 3 loops, 3 triacs outputting live to each loop. If we ever have an element break down and go short you can get by until a replacement is ordered by disconnecting the line and neutral to the short loop and just running on 2 loops, takes a little longer to heat up in the morning but not normally much of an issue.

Our workshop and training room both use type C 32A mcbs

 

[UKMeterman]

One thing to watch out for is relays and others isolating external test voltages. You may need to go round the instrument and follow the mains with a 500V insulation tester, testing L E and N E

 

[static zap]

Any scope for slop tray misfortunes , in customer situ is machine spot on level.
( We had a free machine -that shut off if slops bucket got full. )
Is there a night shift on with no machine access ? ... a method was devised to empty slop bucket by our night shift !
Plenty of water users ?..it leaves little residue !
(may be way off - throwing random thoughts in the mix)
Basic thought .. spilled hot water -somewhere peculiar-triggers RCD.
(other differences with install , mech shock thru water feed)

 

[marconi]

Ndagger: You mentioned the espresso machine is on one rccd of a splitboard. If, as I am exploring, the over current is as a result of spurious triggering of the triacs because of a brief high rate of change of L-N voltage aka voltage transient - say when fridges or freezers start or other motorised equipment - causing too many elements to be momentarily energised which causes a current which exceeds the surge capability of a type B mcb - what one might do is:

1a. Move the mcb for the machine to the other rccb to see if that makes a difference - simple and easy to try.

1b. Change mcb to a type C if wiring and installation method allows - an electrician to confirm.

2. Buy a low pass power line filter and wire it into the mains supply to the machine which would filter out the voltage transients. A suitable power line filter is I think - so don't buy yet - this one:

FN2010-30-08 | Schaffner FN2010 Series 30A 250 V ac 0 → 400Hz Chassis Mount EMI Filter, with Stud Terminals | RS Components - https://uk.rs-online.com/web/p/power-line-filters/1705188/

This would need an enclosure, holes, packing glands and wiring in. Easy to do with some skill with electrical components.

It could be that the machine's own EMI filter has failed allowing these voltage transients to trouble the triacs so this component could be change before buying the LP filter.

I am wondering if there is indeed nothing wrong with the machine. Rather it is reacting to unusual voltage transients on the wiring at the clients. The report that the machine has tripped the mcb immediately on power up and also when idle is interesting.

It would also be worthwhile tightening up all connections L, N and E back as far as the meter to ensure loose connections are are not aggravating the situation and also the other final circuits at the CU.

A bit scatter-gun 'cos I am being nagged to take my wife somewhere now - also check that the temperature sensors are screened and properly connected in case their cabling is allowing spurious signals to be picked-up.

 

[Ndagger]

I've been out all day on call outs so haven't had chance to get any ratings off of the machine but should have a hour free in the morning.

Regarding your suggestions,
1a. I've suggested moving the coffee machine mcb from the left rccb to the right several weeks ago when I thought it could be a accumulation of leakage from other circuits, whilst he hasn't explicitly said yes he will get this done I think he will arrange for that to be done because it's a quick cheap job.

1b. In my last email I did point out that on my type C breaker it hasn't tripped and would recommend he change from type B to c due to the nature of the machine connected to it. He did agree to get this done but also said that as the machine trips during boot up sequence when no elements are in circuit it won't make a difference

2. I have thought about this but based on his sequence of events have put the triacs to the back of my mind for now, he said the machine trips immediately when the rocker makes. This machine is a lot like a computer when it turns on, it displays a lot of info on screen like software rev, a splash screen and other pointless stuff. It takes a solid 90seconds for the "home screen" to be shown and at this point the elements, cup warmer and solenoids start to be energised. I'm awaiting info from the manufacturer on what happens and at what points during this boot up procedure. My understanding is as this machine is computer controlled, for the first 90 seconds after switch on, the only parts energised are the rocker and the pcb. Once booted up the pcb then energizes other parts of the machine with triacs and standard relays but have asked for a flow chart of the boot up procedure from the manufacturer to see exactly what happens behind the black screen of bootup.

And also the first time I returned the machine after not finding a fault in my workshop I thought the original installer may have not tightened one or more terminals in the commando socket and a loose wire may have caused the trip under load so upon return I checked all connections in the socket for tightness, all was fine but haven't checked any terminals in the CU

 

[static zap]

Does the unit have any obvious mains input filter , (separate from pcb)
(also a thorough inspect of mains input cable restraint (if applicable))
Have you done any earth continuity testing
-of kit miss behaving with more than a few mA ?
(like an MFT testers 200mA)
(are any relays involved in initial boot up sequence ?.. as others mentioned L-N path )
.. Monitor supply volts at customers premises as it boots .. (via a commando fly lead ?)

 

[Ndagger]

Only made it into the workshop for half an hour today but made getting a few snaps of it a priority.
Didn't manage to get snaps of the elements in the coffee boilers as they take a hell of a lot of disassembly to get too but I got the boards, filter, transformer, steam element, one of the solenoid coils (The other 14 are same coil) big black transformer and the motor/pump

Not very detailed snaps I know but time was short, lots of breakdowns today!

[automerge]1570822667[/automerge]

The red flat element in one of the pics above is a cup warming element, the top panel of the machine is 2 halves, each half has one of these elements on it so there are 2 x what you can see in the pic

 

[DPG]

It's a complicated beasty.

 

[Ndagger]

Considering it's sole job is to make 60ml of espresso in each go its almost ridiculous right!

Note on the pic of the pcb there's a 4pin header labelled 'WiFi

Mental!

 

[shaun1]

Does the machine self test all the heating elements etc when it is starting up? That could explain the trip on startup.

 

[static zap]

Spotted a few brown splashes , need to keep eye on motor cooling fans ! (Not meggered hot espresso liquid yet)
forgive my lack of enthusiasm -I drink tea.

 

[Ndagger]

Shaun1 - still waiting for info from the manufacturer on what the damn thing does during boot up, it's just a black screen with software revision printed across the centre, what it's doing in the background I have no idea.

Brown splashes are par for the course inside these machines, I take it you spotted the staining on the coil, those are very well potted and generally aren't too bothered by a splash of espresso, normal sign of them causing a trip (especially these 24v ones) are they swell right up

 

[marconi]

Have you noticed how many home produced products are inside this Italian manufactured machine - I have not seen 'Made in China/Japan' so far? The Italians are prolific electrical component makers.

Ndagger - I see that there is a yellow sticker on the side of the power supply informing the voltage can be set to suit the mains. Might be worth a check and comparing with mains voltage measurements I mentioned earlier.

A thought 'out-loud' is that if it is a switch mode PSU, then if the input voltage falls or dips suddenly it will compensate by drawing more current. There is often a significant surge when first powered-up. Once powered up the current waveform has a higher crest factor (= ratio of peak current to average current) which could trouble the type B mcb but not a type C.