2.5mm² is probably fine on a 40A MCB (provided it's not a 10kA version) for faults as the generic max energy let-through should be under 66,000 A²s. You'd still need to make sure that the loop impedances where low enough to get instantaneous operation of the 40A device though - which may or may not be a challenge, depending on the exact device, cable lengths and supply involved.

I've not come across a beast with one input and 4 outputs - do you have a particular device in mind? I would have thought a more usual arrangement would be either 4off single pole devices or a 4-pole device with separate input terminals - that way you could have your 4 16A MCBs upstream of the relay and ensure full protection throughout (including for the relay itself, which may require some study of section 536 and data from the contactor (relay) manufacturer).

   - Andy.

Please understand I'm not a panel design expert -

Perhaps use those 4 outputs to switch 4 small contactors (Which have their own circuits running through them) or use a single N/O or N/C relay to switch a 4 pole contactor with 4 circuits on it. 

If you had to do it the way you've described, I'd see no harm at all in adding circuit protection for those downstream "Circuits", you'd have to be careful with the neutrals though, because those loads would also add up. I'm no theoretical expert on controls, but having worked with controls a bit, it's Definitely not what I'd expect to find or see, so it might create future problems when someone like me comes to fault find or perhaps even pass the new installation. 

Are you responsible for the design?

Thanks Andy.

The relay itself has an input, which is then essentially split between 4 relays. The relays then have their own individual outputs.

I think I’ll go for a breaker after each output, just to be on the safe side. 

Hi,

Switching contactors is one option however the relays are rated at 16A so they can be used to switch the load directly. The manufacturer states acceptable load types (resistive, inductive, capacitive). 

Thanks. 

The 2.5mm cable will be OK to trip a 40A B type on all but the longest circuit, assuming  the voltage drops at 16A are 5% or less.
If it is sensible to fuse or MCCB the loads ? that  is an interesting one. Personally I'd consider it wise, as if not one maybe has to consider odd faults like heater elements shorting to ground half way along. 13A fuses are cheap ;-)

Also be aware that relays have a far shorter life at full load than lightly loaded, and it depends how it will be used - a load tripping on and off every few seconds for a week is a lot worse than switching on and off once a day for a decade. Can the relays be changed or is it a whole module job? You may need suppressors at the contacts as well.

M