isolated supply help!!

[digital_boy]

Guys in a solar PV plant room I have a single phase LV board for power and lighting. This is fed via 2 phases off a solar inverter through a 415 > 230v Isolating transformer.

The earth conductor in the secondary side of the transformer is not connected. Theres a 16mm cpc going from the LV board to an earth bar which is the main earth bar for the plant room.

I cannot get the RCDs to trip on this LV board (assuming due to the isolated supply)? Even on the inverters I cannot get a Ze reading at all as my meters are coming up with no earth connection. I'm again assuming this is a completely isolated supply but how do I get the RCDs to operate? do i connect the disconnected earth up to the neutral in the transformer???

Really racking my head this one, any help would be great.

 

[snowhead]

I get that but now the client is asking questions, we didn't design the system we just maintain it, there are no drawings on site, my main concern is this LV board and how I get round that!

If you are there to carry out an EICR and you have found something out of the ordinary and beyond your knowledge, then as an individual it's not your problem.

Your company needs to employ a Design Consultant familiar with P.V installations to confirm whether the installation is safe and compliant and if not design a solution.
Or if it's outside the remit of your company contract then the customer should employ the consultant.

You cannot just "Get around the L.V board issue"

I'm sure if some people who've responded on here were stood in the Plantroom and actually saw the installation they'd probably have an Aha moment and realise they've seen it before and could explain it.

 

[Rockingit]

If it is a FI , then the installation is unsatisfactory.

You would then be obliged to indicate what work, or investigation is required to address this, so the questions still arise.

Probably of greater note, if this system is being maintained by the OP's company, they really ought to have an understanding of the installation in order to maintain it.

If you have an IT system, but no IFLS then there needs to be a periodic inspection to confirm the integrity of the insulation, which may be 6 monthly or so; therefore this needs to be a normal maintainance task documented and completed by the company performing or responsible for the maintainance.

That may well be the case, I was just highlighting the task in hand and ‘mission creep’. There’s also a part (b) to this, probably, because IT systems are only allowed to be used under skilled supervision. There’s no shame in saying that you need to refer to original design requirements (therein may be a pile of risk assessments…)

 

[timhoward]

Take this with a ton of salt, but I think in your position my conclusions would be:

1 - Write up the EICR as usual, record what you can. Record the IR results including the suspect ones. C2 the low IR. After all, the low IR needs fixing whatever else is going on.

2 - I think I was wrong above. The more I think about this the more I don't understand what possible use the LV RCDs are. As the isolated current flow has no reference to real earth at all, on first fault it's just like joining a wire to nowhere, and on 2nd fault the two wires would essentially connect and the resistance between them would behave like a resistive load. The regs may require their presence even if they don't do anything though?
I think you can prove they function (in abstract) for purposes of report by sticking Line probe on outgoing 'line', and N and E probes on incoming 'neutral'. My thinking is that this 'leaks' test current and should operate the device.

3 - Main question in my mind would be will ADS occur for 2nd fault on a final circuit, where the cpc links both faults and allows high fault current to flow.

4 - As discussed above by others, my understanding is that the regs require a monitoring system. As others have said a maintenance schedule that looks at the monitoring device, or performs every possible IR test would be required. If no device and no signs of maintenance then this is a C2, as someone could be touching exposed metalwork and working on something the other side of the transformer which would normally be safe, but isn't after first fault.

 

[Julie.]

Take this with a ton of salt, but I think in your position my conclusions would be:

1 - Write up the EICR as usual, record what you can. Record the IR results including the suspect ones. C2 the low IR. After all, the low IR needs fixing whatever else is going on.

2 - I think I was wrong above. The more I think about this the more I don't understand what possible use the LV RCDs are. As the isolated current flow has no reference to real earth at all, on first fault it's just like joining a wire to nowhere, and on 2nd fault the two wires would essentially connect and the resistance between them would behave like a resistive load. The regs may require their presence even if they don't do anything though?
I think you can prove they function (in abstract) for purposes of report by sticking Line probe on outgoing 'line', and N and E probes on incoming 'neutral'. My thinking is that this 'leaks' test current and should operate the device.

3 - Main question in my mind would be will ADS occur for 2nd fault on a final circuit, where the cpc links both faults and allows high fault current to flow.

4 - As discussed above by others, my understanding is that the regs require a monitoring system. As others have said a maintenance schedule that looks at the monitoring device, or performs every possible IR test would be required. If no device and no signs of maintenance then this is a C2, as someone could be touching exposed metalwork and working on something the other side of the transformer which would normally be safe, but isn't after first fault.

An RCD is an acceptable method of ads for an IT system for detection of a second fault, it is permitted, and one of the recommended methods in the regs.

It can also be used as a disconnection device on first fault for IT systems having high impedance earthing ~7k ohm or lower.

(This could be why the OP is seeing some "earth fault" on one of the circuits, but perhaps unlikely)


The regs didn’t actually mandate a first fault monitoring system, but do now.

Therefore you could have a manual periodic check on an older system, but on a new system it must have an audible or visual IMS.

 

[pc1966]

I strongly suspect there is no need for it to be IT, but without the original design info we don't know.

The low IR needs fixing anyway no matter that system is in use.

Raising it as 'FI' might be one way to force a resolution as if it is not regularly maintained /skilled supervision, or has IMS system, then it is not compliant with the regulations. So either it stays IT but has some means to check insulation defined, or it is converted to TT or similar after reviewing the original design so usual RCD behaviour for ADS, etc, can be expected.

 

[timhoward]

An RCD is an acceptable method of ads for an IT system for detection of a second fault, it is permitted, and one of the recommended methods in the regs.

I believe you! But the thing I'm not grasping is how it would work. I think I'll start a new thread...