Air Source pump tripping RCD

[SQUIGGLIES]

Keeping this brief.....................New dwelling.....TT system with 100mA S type rcd protected supply to local board with 30mA RCD. Freelance "engineer" to commission Heat-Pump, as original company disappeared before installation. ASHP worked for a while then tripped the 100mA RCD not the 30mA. Both RCDs tested at nominal trip current and ramp test.....both passed. Selectivity obviously working fine. The supply works with a 3KW kettle and will operate with the ASHP if the earth is disconnected at the ASHP.
There is a very healthy earth to the DB.

Theories and similar experiences appreciated...........................

 

[snowhead]

Has anyone tested the Earth leakage current at the ASHP and also at the D.B / C.U it's fed from?
What else is fed from the Local board with the 100ma S type?

 

[Avo Mk8]

Snowhead beat me to it - typing too slowly!

Sounds like the ASHP has developed a leak to earth. Moisture somewhere?
Have you IR tested the circuit and pump?

Sorry no similar experience - my leak was glycol not electrons

 

[SQUIGGLIES]

Hi Thanks for your input. There is a workshop and stables fed from the same protected circuit. This was the first point of call and the circuit was isolated (double pole) with no effect on the outcome. The AC side of the solar array was also isolated even though this is fed into the main phase blocks through its own DB. Earth leakage at the DB was 3.1mA, I did not test the ASHP as I presumed that would be part of the "engineer's" procedure. Ramp test for the 100mA S type RCD 72mA, and for the 30mA was 21mA. Supply cable between 100mA RCD and DB was clear and fine.

 

[SQUIGGLIES]

Hi Avo,
That would be the initial thought, but surely the 30mA RCD would trip first as it did in all of the tests? The supply cable from the DB to the ASHP is also fine by the way.

 

[Avo Mk8]

Well I thought that your comment that disconnecting the earth from the ASHP allowed it to run was very significant.
I don't have any suggestions as to why the 'wrong' RCD tripped!
But I think that is secondary to finding the cause of the problem.

ASHP can generate a lot of condensate, depending on how hard its working and relative humidity, although at this time of year not so much. I was just wondering if earth leakage could be self-inflicted by the ASHP on itself! Eg because of a drainage problem for example.

 

[pc1966]

Was it a 30mA RCD (that DP switches), or a RCBO that is not neutral-switching?

Unless neutral is also isolated then you can see RCD selectivity fail due to enough current on neutral to imbalance the upstream one.

 

[SQUIGGLIES]

Well I thought that your comment that disconnecting the earth from the ASHP allowed it to run was very significant.
I don't have any suggestions as to why the 'wrong' RCD tripped!
But I think that is secondary to finding the cause of the problem.

ASHP can generate a lot of condensate, depending on how hard its working and relative humidity, although at this time of year not so much. I was just wondering if earth leakage could be self-inflicted by the ASHP on itself! Eg because of a drainage problem for example.

I am totally with you on that Avo. It makes no sense as the supply cables are all sound. I am hoping someone sees this who has had the same experience. All I can think of is the 100mA RCD has been damaged in a way I have never seen. It tests fine. It is an AC type as it was installed in 2018. Some HVAC guys I spoke to say they specify B Type RCDs as matter of course and that should come from the manufacturer or engineer. Although these RCDs deal more with "DC-blinding" where as in this situation, not tripping is not an issue.

 

[SQUIGGLIES]

Was it a 30mA RCD (that DP switches), or a RCBO that is not neutral-switching?

Unless neutral is also isolated then you can see RCD selectivity fail due to enough current on neutral to imbalance the upstream one.

Hi PC, it is a double pole RCD main switch.

 

[marconi]

I suspect the ASHP's inverter circuitry and associated EMC filter are producing an earth leakage current which may extend to higher frequencies than the mains 50Hz. The first thing to do is look at the ASHP's installation manual to discover what type of RCD is required. Please post full details of the ASHP so we can look up too. A quick way to find out would be to phone the ASHP manufacturer's technical help line.

Here is a short article which gives an introduction to what I suspect is happening:

Heat Pumps – Type F or Type B RCDs? - Electrical Contracting News (ECN) - https://electricalcontractingnews.com/news/heat-pumps-type-f-or-type-b-rcds/

I am not at this stage by showing you the link saying that the solution is a Type F or Type B - we need to look at the manual or contact the manufacturer first.

Note also that one cannot place a Type F or Type B downstream of a Type A or AC so it is likely your ASHP may need a dedicated supply taken from the meter output.

 

[marconi]

Further to my last: please would you use your MFT to measure and then report the Ze ( not Zs) so we can assess the site’s electrode resistance Ra. Do this at the db feeding the ashp.

 

[SQUIGGLIES]

I suspect the ASHP's inverter circuitry and associated EMC filter are producing an earth leakage current which may extend to higher frequencies than the mains 50Hz. The first thing to do is look at the ASHP's installation manual to discover what type of RCD is required. Please post full details of the ASHP so we can look up too. A quick way to find out would be to phone the ASHP manufacturer's technical help line.

Here is a short article which gives an introduction to what I suspect is happening:

Heat Pumps – Type F or Type B RCDs? - Electrical Contracting News (ECN) - https://electricalcontractingnews.com/news/heat-pumps-type-f-or-type-b-rcds/

I am not at this stage by showing you the link saying that the solution is a Type F or Type B - we need to look at the manual or contact the manufacturer first.

Note also that one cannot place a Type F or Type B downstream of a Type A or AC so it is likely your ASHP may need a dedicated supply taken from the meter output.

Hi Marconi, This was the piece of the puzzle it felt like I was missing. The company who were meant to be installing the renewables went AWOL. I had no information beyond the KW loading of the unit and some "monkey see monkey do" instructions regarding the control cables from the water cylinder and heating centre. RA on the day of testing was 11.02 Ohms.
Another avenue I was keen to explore was finding out if the delay mechanism for S Type RCDs is mechanical or analogue and if this could possibly become damaged by a malfunctioning ASHP? Thanks for your input.

 

[marconi]

I do not think the Type S RCD has been damaged. The S type may be more sensitive to the waveform of the earth leakage current than the 30mA RCD and thus trips first. Let us know what the ASHP manual says about type of RCD. You tested them I believe and found they operated correctly?

 

[SQUIGGLIES]

I do not think the Type S RCD has been damaged. The S type may be more sensitive to the waveform of the earth leakage current than the 30mA RCD and thus trips first. Let us know what the ASHP manual says about type of RCD. You tested them I believe and found they operated correctly?

There was nothing in the paperwork that accompanied the ASHP that referred to the supply. Also, when the "engineer" was commissioning the unit, the manufacturer's technical help-line never mentioned the type of RCD, even though they were made aware of the issue.
Both RCDs tested exactly as I would expect both on ramp and time. Selectivity was also fine with the 30mA tripping first every time.

 

[marconi]

Please send me the details of the ashp and I will do some research.