DC through armoured cable

[SolarCity]

Just wondering what other installers' thoughts are regarding DC and armoured cable.

I have an install coming up where we have a fairly long cable run. I would tend to install the inverter at the array and run a long AC cable back to the CU. I know some people have actually run the DC cables back to the CU itself and installed the inverter there. To me this seems like a bad idea as there is the potential for shorting out the steel wire armouring down to the DC, which would introduce a DC current to the main earth.

I'd be interested to hear any thoughts.

 

[BruceB]

Reasons for putting dc through a long swa cable:
- dc voltage likely to be higher than ac voltage, therefore current lower, therefore power loss lower for same size of cable.
- no problem with ac voltage drop causing exterior inverter to cut out due to over-voltage. More important with G59 as allowed over-voltage is lower.
- if there is an earth fault in the dc cable, then it would be detected by the inverter RCMU and shut down. Low level earth faults would not be detected on an ac cable unless rcd protected.

Reasons for not doing so:
- none really

 

[SolarCity]

- if there is an earth fault in the dc cable, then it would be detected by the inverter RCMU and shut down. Low level earth faults would not be detected on an ac cable unless rcd protected.

Fair enough, the inverter would shut down. But the fault to the earthed bonding on the SWA would remain. Surely this is not desirable?

 

[BruceB]

It puts a dc potential onto the earth in the case of a fault, but unless there was a second fault somewhere that provided a dc circuit then it would not cause any current to flow. So a single dc fault is not immediately dangerous.

There are others here who routinely operate in the commercial field who might have a view.

 

[Gavin A]

Fair enough, the inverter would shut down. But the fault to the earthed bonding on the SWA would remain. Surely this is not desirable?

as bruce says, it becomes a separated circuit with no return path, so no danger. We've probably used SWA on the DC side of around 50% of our installs as we prefer to run the SWA down the outside of the house from the roof, and I prefer the mechanical protection this gives.

 

[SolarCity]

Thanks for the info.

I suppose it is no different to installing on a metal roof which is deemed to be extraneous and therefore bonded. Just doesn't seem right. Wanted to run it by others to see if they flagged anything up. For this installation, it is desirable as we have no means of practically housing an inverter near the array.

 

[Earthstore]

Couple of questions here.
1 Does the DC cable need to be earthed?
2 If so, can you not TT it the "extraneous" end, keeping it completely separate from the AC therefore removing any potential for DC entering AC?

Only trying to help here?

 

[Gavin A]

Couple of questions here.
1 Does the DC cable need to be earthed?
2 If so, can you not TT it the "extraneous" end, keeping it completely separate from the AC therefore removing any potential for DC entering AC?

Only trying to help here?

1 - arguable, we don't earth it on isolating transformer inverters because IMO it's safer not to as it's an isolated circuit, but do on TL inverters after a debate on here IIRC about it when I realised there was some logic to it.

2 - Sounds like a bad idea to me to introduce an alternative earth path, plus I've no wish to have to go banging earth rods in again if there's not a very good reason for it.