It looks a poorly designed installation, and should not be designed or installed like that these days.  A short circuit on the 32 amp socket outlet circuit might trip two or more MCBs, thereby interrupting supply to other loads. 

The single RCD also protects a possibly excessive number of final circuits.

In the case of an existing installation, if it works reliably without unduly frequent trips, then I would be inclined to accept it.

The 6mm cable is not at risk of overload, the 32A trip that covers it sees to that. The 25mm main similarly is covered by the 100A fuse.

But, there is a lot of load in those boxes compared to the supply to them, especially the 2nd one if you use the textbook approach.

So are there mitigating factors that may justify a non- standard calulation ?-

Are there 2 socket circuits not because they are needed to serve a massive heavily loaded area, but instead perhaps serve only a small space but are used to keep IT equipment on its own circuit or to provide some other segregation by room or load type?

If so then they may not be as heavily loaded as one would assume.

Do the 2 water heaters never come on together  for some reason or maybe have very short duty cycle? - it may be alright in practice if they are only both on for a few minutes at a time. Are the heaters taking the full 16A each or is that the breaker size but really they are much smaller load ? 

Or is it all loaded to the max and tripping every other day ? If so it is just wrong, and needs some cable and breakers changing for the next size up. It may have been added to in stages over the years without too much 'bigger picture' thinking.

But these are the questions I would be asking.

Mike.

Why are you asking? Has there been "nuisance tripping"? Are you doing an EICR?

I don't get too excited about MCBs in series. If the design is right, nothing should be overloaded. Short circuits should be a rare event.

So the purpose of this is for an EICR for part of a commercial/industrial site, but not carried out by myself, I'm trying to understand what the best solution would be 

This part of the installation is currently without any tenants but the Site manager is wanting to fill his units up hence the EICR and getting the installation made safe. So, as there isn't much usage at the moment it's hard to determine if the current setup is reliable, although they have reported some tripping in the past. 

There is a second part to this installation with 3 and 4 boards in series non of which have any discrimination. As mentioned in a previous message, it's evident there has been a multitude of additions over the years.

I am working with the site owner who will be upgrading the incoming supply to the whole site soon, roughly 20 businesses. So a new supply location and new cables will be supplied to all sub-mains on site. So trying to see if it's worth convincing the particular site managers to upgrade their installations at the same time, only if justified.

Discrimination is more a nice-to-have rather than a blanket requirement - there are very many cases where it's not achieved - "split" CUs where an earth fault on once circuit will trip the RCCB and so cut the power to half the house - between 13A fuses in FCUs and upstream 32A MCBs for faults - between pitch and caravan RCDs - the list goes on an on. Usually it's a balance between convenience and cost  - ask a customer if they want full discrimination and they'll say yes of course - explain that it'll add an extra £20/way for their 48-way DB and they often change their mind! Not so long ago entire installations, large as well as small, had a single up-front ELCB.

  - Andy.

AJJewsbury said:

Not so long ago entire installations, large as well as small, had a single up-front ELCB

Indeed, and not only that, one of the guidance notes a few years back said it in itself did not break that discrimination part either.

I doubt that opinion would be stated nowadays but it was quite explicit back then and I admit it surprised me a little bit (but not entirely) at the time when the front enders were all the rage.

I think discrimination becomes much more important if the upstream protection isn't easily accessible - e.g. in an industrial estate or flats where upstream is in a locked cabin where it will take two working days for the landlord's managing agent to attend with a key.

I will certainly ditto that one, wally

In fact these days I often opt for RCBOs for that reason