UK RCDs/RCBOs and DC Interfereance

[Strima]

I've just carried out a periodic inspection on a building on an old RAF/USAF site. Commercial single story building.

Wiring system is around fifty years old run in a mix of trunking and steel conduit.

Three phase supply, TN-S from local transformer around fifty meters away, underground supply in a mix of trench and conduit.

Plant room has busbar system feeding this building and three supplies outgoing to other businesses the other side of the road.

No lightning or surge protection.

Ze 0.15 ohms, PFC of 3.7kA.

IR tests of cables all above 4.45 MegOhms.

All R1+R2 well within requirements, all sub 1 ohm.

Lighting, sockets, fire alarm and water heater supplies.

Installation in good condition, roughly 12 LED panel lights, and a couple of switched mode power supplies.

Chint TPNE board, all single phase circuits.

All dead tests passed easily.

As soon as I try to do Zs the RCBOs trip out, this is using the low current no trip setting on a Megger 1552. Works fine on a different installation as I've just tested mine at home so it's not the tester.

When doing time tests on the RCD part they trip before the test can be completed, basically as soon as the meter starts to inject current it trips out the RCD.

All are Chint 61009 30mA AC type RCBO, B curve, 16, 20 and 32 amp.

I have checked for earth leakage on the main earth, no AC current detected but I am getting six amps of DC.

Also getting six amps DC when the DB tails are clamped but there's nothing on the installation that can generate this much current, the with DB isolated I still get DC current on the main earth.

I'm unable to turn off the other outgoing supplies.

Other buildings near the transformer are an old ammo bunker, some mad bloke who does some radio stuff and has PV set up on his roof, but nothing big enough for this amount of current.

The customer did mention that there's a solar farm just around the corner. Could this solar farm be pumping DC back into the HV/LV networks and could this be the cause of this large amount of DC detected on the earth?

 

[Lucien Nunes]

Any current that flows in the earthing system with the installation isolated is not residual current from the installation. The RCD should only respond to residual current, so whatever is going on with circulating currents in the earthing system, although perhaps symptomatic of something else, should not be causing abnormal RCD operation.

6A DC residual current in the tails does seem odd though. Have you checked the operation of the clamp meter with a representative DC current to ensure it is reading correctly (e.g. not 6mA or just displaying that number when anything is present). Beware the possibility of it misreading due to RF currents from the radio installation if it is a transmitter, although it is unlikely to remain keyed up for the duration of your tests.

 

[pc1966]

If it is 6A stray DC it could explain the RCD trip behaviour. Though AC types are often blinded by DC I suspect the MTF kicking in with a ~6A step would trip many devices.

But where the DC is coming from is stranger. To get 6A DC from a PV farm hundreds of metres away would be remarkable, it would need a huge DC flow in the Earth and there would be plenty of other issues going on! The only PV system I have ever dealt with had the PV panels to the inverter DC circuit as an isolated supply (IT) with a earth resistance monitoring arrangement, I guess others would do something similar?

To me it sounds like something like a welder being used, or other high power DC equipment with some sort of a fault, that must be pumping a whole lot of current in to the local CPC setup / building structure.

Or maybe as Lucian suggests it is RF screwing with both the clamp meter and RCD testing.

 

[Strima]

I've worked on a lot of mobile phone sites and other radio masts and I've never seen that level of DC leakage.

I will be returning with a different clamp meter, but my settings were correct and it was six amps, I checked around five times just to be sure.

Hopefully I can get there during working hours to have a quick check in some of the other buildings but I've asked the customer to pass this onto the site owners maint team to see if they have had any other problems.

I will be trying an A type RCD but this is melting my head a little, especially on a Saturday.

Thanks.

 

[Marvo]

If you're seeing the 6A DC when you clamp both tails as a live test then it's deffo something within the installation. It's worth remembering that any waveform the tester doesn't understand as AC it will identify as 'DC'. This would apply to halfwave AC leakage for example. Personally I'd put a portable clamp-on scope or clamp-on PQA on it to see the shape of the waveform, this might give more of a clue to its origin.