Does a 3 Phase socket supplying a Portacabin's 30mA-protected DB still need its own RCD protection?

[Mark42]

This month I'm building a remote lab for specialist chemistry work, in a self-contained 'Portacabin' type building.
The cabin will have a new, Hager 3 phase board, with an 30mA RCCD main switch. It's only a single small room so I'm not bothered about the possibility of the whole installation tripping (one of the ceiling lights will be an emergency unit).
As the building's portable, its Hager board will be supplied from a 32A IP67 5-pin appliance inlet on the outside wall.
This will be fed from a new local external power post with a 5-pole IP67 32A socket. A short rubber extension lead completes the connection.
Here's the problem: How do I supply the power post? As it's outside, with a 400V socket continually being rained on, 30mA protection feels like a good idea. But then there would be two RCDs in series. Not good.
And if I do protect the power pole socket at 30mA, there's no real need for the upstream protection in the cabin. But then any tripping would mean a walk back to the potentially locked building where the main DB is.
I want the flexibility of 'plugging in' the cabin, so don't want to hard wire anything.
Is there an obvious solution? Am I missing something?

 

[cliffed]

May use a time delay @ power post, for correct selectivity.
Also rcbo’s in your DB If required.

 

[pc1966]

An outside socket as you describe should be protected but it leads to the upstream tripping issue and access to reset it.

You can get self-resetting RCDs from Schneider witch is one possible solution.

How "temporary" is this building? It might be better simply to have a length of SWA cable run in so no feed RCD needed, and accept it is a half-hour job for a sparky to disconnect when it is eventually moved.

 

[telectrix]

another possibility is supply socket without RCD upstream, but use a socket theat can not be used for anything else. e.g. a padlocked on plug (if such a thing exists), or inside a locked enclosure.

 

[Lucien Nunes]

Cable from the DB to socket in SWA or something that does not need RCD protection. Use a socket outlet with a built-in RCD compartment so that the socket is protected but the protection is accessible to reset by someone using the cabin. Put another RCD in the DB so that its installation is compliant in its own right. The objection to having multiple RCDs cascaded is a practical one rather than electrical.

 

[davesparks]

May use a time delay @ power post, for correct selectivity.
Also rcbo’s in your DB If required.

But that wouldn't provide 30mA protection at the external socket, which is what is required for protecting sockets.

 

[Mark42]

... How "temporary" is this building? ...

Temporary enough not to need planning consent
I don't want to muddy any waters by wiring it in directly ...

 

[Lucien Nunes]

My 'put another RCD in the DB' of course is referring to the Portakabin DB, but I figure from the reactions that this is clear. A time-delay RCD could be used in the building DB if the Zs is too high for the socket circuit to be adequately protected by the MCB (I recall some of the OP's buildings are themselves fed via long distribution circuits). Then there would be three RCDs:
Delay RCD in the building DB protecting the cable due to high Zs
Normal RCD in the socket providing additional protection to the load
Normal RCD in the Portakabin protecting its installation if it is connected to any other source of energy.
But all OK in my book

 

[Mark42]

another possibility is supply socket without RCD upstream, but use a socket that can not be used for anything else. e.g. a padlocked on plug (if such a thing exists), or inside a locked enclosure.

That's an interesting idea. I wonder if that would be technically compliant. i.e. When is a socket for portable equipment no longer a socket? Answer: When it's in a locked box. I wonder?

My 'put another RCD in the DB' of course is referring to the Portakabin DB, but I figure from the reactions that this is clear ...
Normal RCD in the Portakabin protecting its installation if it is connected to any other source of energy ...

Of course. Quite clear. Thanks.

... A time-delay RCD could be used in the building DB if the Zs is too high for the socket circuit to be adequately protected by the MCB (I recall some of the OP's buildings are themselves fed via long distribution circuits).

Crikey, I have a stalker! Well remembered
Yes they are very remote buildings, but there are additional rods everywhere. That's another conversation.

... Then there would be three RCDs:
Delay RCD in the building DB protecting the cable due to high Zs
Normal RCD in the socket providing additional protection to the load
Normal RCD in the Portakabin ...

Splendidly engineered but I'm paying for this!
I'll use buried SWA and skip item 1.
Now I need to find a 32A three phase IP67 socket with integral 30mA RCD protection, without spending more on this that the lab cost! EBay is my friend (Unless anyone on here has one for sale?)
[automerge]1596189766[/automerge]
UPDATE
Not as bad as I expected. Have 4-pole RCDs come down in price?
Although it's against my usual nature, I'll buy the more expensive IP67 one as the plug exits downwards, which is neater and less likely to get snagged.

Job sorted. Sometimes this forum is brilliant! . Thanks gentlemen.