Update 30 Nov - So, having had apparently fine operation since installing the SSR. The ccgx suddenlty started frequently and apparently randomly restarting, then just wouldn't come on at all. Turns out the fuse holder on the + supply was cracking and the connection was bad... Hence I'm accepting JohnC's answer. My guess is something about when the contactor switched was exacerbating issue.
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Update 19 June - I ended up installing a Fotek SSR-40 type solid state relay in place of the contactor, driven by the battery bank. So far so good! Unfortunately it means I'm not able to confirm any of the answers given below, though I'm sure they're right!
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Please see image below for overview of the system.
The CCGX relay output is used with generator control to switch a charger based on SoC. Mains supply voltage is unreliable/variable, hence this configuration.
I've noticed often (but not always) the CCGX will restart itself when the contactor is switched off. This has been the case for perhaps 2 years now (I keep "meaning" to do something about it!).
Today, there was an apparent further progression of this problem, which I have not seen before. The system became stuck in a loop: contactor switches on -> small delay before xantrex starts charging -> ccgx trips/restarts -> contactor switches off due to ccgx turning off -> repeat.
I was able to break the loop by turning off the mains briefly until the ccgx had restarted and settled, then turing mains back on.
In this case the loads on the Multiplus where quite high 3.5kVA at the time.
Inverter/battery cables are undersized - 35sqmm (Again, something I've been meaning to rectify!).
I realise I should probably sort this out first, but I wondered if anyone might have any further ideas?
Many thanks