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Posted By Topic: Caravan with three supply sources

MitchB
Aug 01 2019 11:51

I have a customer who wants to build a caravan with three sources of supply: 230v mains supply, plug in generator supply and inverter supply.

There's nothing in 3001:2008 that details this scenario. The closest is 3.4.4.3, "Dual-Supply Systems", although this is a triunal supply being proposed.

He is proposing having the inverter (powered by the 12v) powering three dedicated 230v single phase 10A outlets, which are electrically separate of the other two supplies.

The (external) generator and 230v mains supplies will be fed from two respective appliance inlets, to a changeover switch as per Figure 3.1. This bit is no problem.

My feeling is that the inverter fed power outlets need to be isolated by the main switch (changeover switch). I'd likely get K&N to make a "mains-off-gen-off-inverter" 5 position switch. Can anyone see any issue with this (or with the original way proposed by the customer)?
   

AlecK
Aug 01 2019 12:36

with separate inlets, for a 3-source system you'll need a 3-source selection switch
One with appropriate voltage rating as well as isolation rating will be expensive; so probably a 5-position, with an "off" between each pair of selectable sources

For the wiring just treat each pair of sources as shown for the '2 source' scenario in the standard.

To make it simpler, leave out the 2nd inlet, and just plug the (single) supply lead into the genset.

Even better, if the inverter is to supply only dedicated circuit(s), you don't need any c/o switch at all. However I would pass the inverter supply via an isolating switch co-located with the main switch for shore power. People generally expect something labelled "main switch" to control everything, so logic says keep all such device together and label them carefully.

Either way be careful with selection of both genset and inverter, so that RCD protection is effective for all final subcircuits regardless of source.
   

MitchB
Aug 01 2019 12:59

Thanks Alec, you've essentially confirmed my thoughts.
With regards to the inverter for dedicated circuits, 3001 doesn't appear to address that at all. I assume you've lifted that from 3000:2007 Part 7?
   

AlecK
Aug 01 2019 13:54

no.

Strictly speaking, such an arrangement is not part of an electrical installation, as it is completely separate (other than being sourced from a battery that may have some sort of charging system connected to it. It's basically no different from those automotive product inverters that plug into a cigarette lighter socket, or for modern vehicles are built into the vehicle itself; and provide a 230 V supply.

Noit being part of either an electrical installation or a connectable installation; it doesn't have to comply with the Standards cited by ESRs 59 & 60.

It only becomes part of the installation when there's a direct connection between them, as fo0r example setting it up as an alternative supply to the "shore power" of a caravan or boat.

But even when not required to comply,. it still has to be electrically safe. Having a master switch for this independent system is simple common sense. Noting that the function could be performed either on the a.c. output or the d.c. input; and that due to the typically significant losses in inverters even with no load; switching the d.c. is generally a better option.

Co-locating that with the switch that performs a similar function for the other electrical system is simply good practice, and will be part of the new guidance in next edition of "3001".