2 supplies to property and the one trips the other out!!

[dlt27]

Hi, thanks for any advice.
I’ve got a local farmer who has single phase to the farmhouse and 3 phase to the farm and dairy etc. The problem he’s got is when he turns the milking parlour on the house RCD trips. I am a bit stumped as to why and how to remedy. I was thinking when he switches the large load on it must be something to do with that. I can’t see any borrowed neutrals etc between the 2 independent supplies anywhere. Just wondered if anybody out there had any ideas. Many Thanks

 

[telectrix]

is the single phase a separate supply or is it 1 phase supplied from the 3 phase?

 

[Pete999]

Hi, thanks for any advice.
I’ve got a local farmer who has single phase to the farmhouse and 3 phase to the farm and dairy etc. The problem he’s got is when he turns the milking parlour on the house RCD trips. I am a bit stumped as to why and how to remedy. I was thinking when he switches the large load on it must be something to do with that. I can’t see any borrowed neutrals etc between the 2 independent supplies anywhere. Just wondered if anybody out there had any ideas. Many Thanks

Is the SP Supplied from the TP and N SUPPLY?????

 

[timhoward]

I've been caught out by a farmhouse supply being abandoned at some stage and a new sub-main feeding it from a different building. So verifying it is two supplies is first step.
Also interesting to know earthing arrangements at each.

 

[Rockingit]

Is it actually 3ph or is it split 2ph? The latter is common on farms. Either way, my guess is that it might well be some kind of weird volt drop/surge issue caused by the dairy firing up. I’m rural, very rural Somerset and theres a noticeable drop in energy (lights dim etc) at milking time!

 

[dlt27]

It’s 2 separate supplies. The farm used to be all single phase, however the farmhouse ( overhead) was left on original supply and a new 3 phase supply was installed for farm (underground supply). I think they’re fed from same transformer. Thanks

 

[timhoward]

if logic applies then the rcd should not trip with all circuits off at the house. So narrowing it down might be an option?

 

[snowhead]

When did the fault start, was it immediately after the 3ph supply was installed or has other work been done since?

If none of the above then the fault is probably a bad connection on the main Neutral at the source where both supplies originate, the transformer, if that's the point the 2 separate.

 

[Rockingit]

When did the fault start, was it immediately after the 3ph supply was installed or has other work been done since?

If none of the above then the fault is probably a bad connection on the main Neutral at the source where both supplies originate, the transformer, if that's the point the 2 separate.

I was thinking along the lines of neutral distortion / harmonics etc myself. You can find some really noisy pumps and compressors in farm stuff!

 

[pc1966]

Always start by doing a global IR on the house to see if it could be N-E leakage.

The other thing to remember is RCD trip on current imbalance. Usually the capacitance L-E only leads to a couple of mA of current due to the dV/dt of a 50Hz sine wave. It is possible that a big drop in L volts when the machine kicks in is causing a much higher dV/dt so just enough current to trip the RCD.

So for that do the usual due diligence of ramp-testing the RCD to verify it is tripping normally (no load(s)) and then use a current clamp meter with mA resolution to get an idea if the existing arrangement is a mA or two, or close to 10-15mA from lost of capacitance (usually mains filters on fancy electronics).

 

[Strima]

Loose neutral on the supply side? What's the voltages like on both installations?

 

[dlt27]

Thanks for all the advice. I’m going back Monday so will report back as soon as I’ve done IR tests etc. I also have a power quality analyser so might put that on it for a week as well.

 

[marconi]

Thanks for all the advice. I’m going back Monday so will report back as soon as I’ve done IR tests etc. I also have a power quality analyser so might put that on it for a week as well.

Hello. I wondered if you were able to visit and what more you discovered?