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steve 27

strange RCD tripping situation

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steve 27

Hi All, 

 

first time poster and hoping someone can give me some advice. 

 

End of last year connected and energised an air source heat pump, run from a 18th edition consumer unit. Customer wasn't happy as the kit (supplied by him) just cycled from freezing up to defrosting itself with the hot water it had produced. No technical support from the manufacturer, so he bought a replacement unit from a different manufacturer. A couple of months later the new unit is fitted and connected using the existing wiring and on first commissioning it the RCD 'protecting' that side of the DB trips. A new HP is delivered and fitted and has exactly the same issue. Tried supplying the unit from a separate 2 way DB, bypassing cabling in house with a temp cable, only heat pump and control circuit connected - same problem. So today I tried a different brand of RCD, in the separate DB and the unit works fine!! replace the original RCD module into the house DB and the trip returns, temporarily fit different brand into main DB and the unit runs fine. 

original RCD - 2 pole proteus type A

replacement RCD wylex type a

both 80a 30ma

both ramp test at 29ma

all trip times fine

new supply, pme 

controls are on same side of board as pump supply and use a bus system so 'no' voltage

IR reading for supply cable to HP tested fine on all three tests (with hp not connected)

I originally thought perhaps a proteus rcd problem, have now tried 3 different proteus modds all with the same result

but it dawned on me earlier that the proteus one could be fine and the wylex is faulty!!

completely baffled! Can't come up with a situation where one make is fine and another isn't when they appear to be working at the same trip level.

 

anyone any ideas? tia

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Murdoch

I suppose the obvious question is does this circuit require RCD protection?

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SPECIAL LOCATION
2 hours ago, steve 27 said:

run from a 18th edition consumer unit.

 

What exactly are you referring to as an 18th edition CU??

 

Which may help others to understand all factors that could cause the RCD to trip?

 

:C

Edited by SPECIAL LOCATION

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kerching

Is it i fed via a contractor and the contacts don't break at same time?

live /neutral breaks a split second before neutral/live

RCD sees imbalance

RCD sulks

 

 

when it worked was unit powered in a different sequence so didn't rip?

just curious

 

oh, WTAFF is an 18th edition board!

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steve 27

sorry guys, dual rcd metal clad board. then two way garage type metal clad as a temporary attempt.

 

The unit  powers up from a central control panel so the sequencing is all internal and the same each time,  one rotary isolator for unit one switched spur for controls. Rotary first then controls switched on manually.

 

Nice thought with the contactor

 

 

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Murdoch
19 minutes ago, steve 27 said:

sorry guys, dual rcd metal clad board. then two way garage type metal clad as a temporary attempt.

 

The unit  powers up from a central control panel so the sequencing is all internal and the same each time,  one rotary isolator for unit one switched spur for controls. Rotary first then controls switched on manually.

 

Nice thought with the contactor

 

 


why does the circuit need RCD protection ?

 

this needs sorting first because dual RCD boards are the work of the devil and the customer will be losing power to some of their circuits ....

Edited by Murdoch

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steve 27
7 minutes ago, Murdoch said:


why does the circuit need RCD protection ?

 

this needs sorting first because dual RCD boards are the work of the devil and the customer will be losing power to some of their circuits ....

 

Hi Murdoch,  cables less than 50mm in wall and manufacturers requirement.

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Murdoch
11 minutes ago, steve 27 said:

 

Hi Murdoch,  cables less than 50mm in wall and manufacturers requirement.


well ... I would consider rejigging the CU , and if you can’t put this device on its own RCBO, remove one RCD and put the circuits it covered on separate rcbos 

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SPECIAL LOCATION
1 hour ago, steve 27 said:

sorry guys, dual rcd metal clad board. then two way garage type metal clad as a temporary attempt.

 

 

Not sure how any dual RCD board complies with 314.1 (i) ?

 

You many need a CU arrangement with better "division of installation" to remove any residual leakage issues from other circuits sharing one RCD.

:C

 

 

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kerching

Dual RCD board does not and NEVER HAS complied with the regs

end of!

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Murdoch
13 minutes ago, kerching said:

Dual RCD board does not and NEVER HAS complied with the regs

end of!


yet people still fit cus with single up front RCDs 

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Fleeting
1 hour ago, kerching said:

Dual RCD board does not and NEVER HAS complied with the regs

end of!

May be under previous Editions but the 18th does permit this providing certain Conditions are met, Regulation 531.3.5.2

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ProDave

This must be the mother of all "fault find by substitution" threads.  I counted THREE different ASHP's tried here.

 

Even when this is working, it is an outside unit, there is potential for damp and condensation, so I would not feed one from other than it's own rcd or rcbo.

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SPECIAL LOCATION
2 hours ago, Fleeting said:

May be under previous Editions but the 18th does permit this providing certain Conditions are met, Regulation 531.3.5.2

 

(1)   Any Actual solutions to the OP's question re "Strange tripping"?

 

(2)  That doesn't actually say a dual RCD board is compliant...

 

Rather that in certain circumstances more than one circuit may use a shared RCD..

 

providing conditions such as 531.3.2..

which then also refers to 314....

are met..

 

Which are NOT difficult to achieve with a few RCBO's to minimise inconvenience..!

 

 

p.s.   

(3)

Not sure you actually stated how long you consider premature to be in a domestic single entry garage?

 

I am interested to learn...

 

Guinness

 

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binky

Proteus - spawn of the devil :^O

 

It may be that the 2 RCDs are different types, but either way I would be looking to put the HP on it's own cct not sharing a RCD with any other cct.

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binky
On 03/05/2021 at 22:17, Fleeting said:

May be under previous Editions but the 18th does permit this providing certain Conditions are met, Regulation 531.3.5.2

 

What we have discussed and concluded on here is that 2 RCDs is not minimising the inconvienience of a fault on 1 cct by allowing it to affect multiple ccts shared with the same RCD, ergo, to comply with which ever reg it is, we should only ever really fit RCBOs as these are widely available from any wholesaler and minimise any issues. 

 

It's a bit like we worked out that sharing a solar system with an RCD protecting multiple ccts is also dangerous. Solar inverters are allowed upto 3 seconds to shut down in the event of loss of mains supply, so you could have run over your lawn mower cable and be getting belted for far longer than RCD trip times allow. In reality most solar inverters do trip out in times similar to RCDs, but as it isn't guaranteed, you can't rely on that to happen ergo inverters need to be on a separate RCD. In my opinion avoiding RCDs at all on inverters, by running cables to comply with that, is a better option altogther due issues with some inverters and earth leakage, plus most inverters have some form of RCD protection built in anyway.  

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