Just to ask the obvious.....why not just install the AFDD?

If the concern is nuisance tripping of the RCD element in a combined protective device, you can install a standalone AFDD module in a mini din-rail enclosure adjacent to the DB. Then the CPD on the DB can be an MCB with an accompanying risk assessment for the omission of RCD protection on that circuit.

There is also a concern that the AFDD is susceptible to nuisance tripping but I'm not sure on the statistics of AFDD performance.

I'm not sure you can buy an AFDD that does not include the RCD function, certainly not easily from most makers.

there may be a very limited no of ways to have AFDD without an RCD.

What Complimentary Network Services are so vital that they have to say on if the building is no fire - and is a UPS or redundant suppplies an option ?

Mike.

Hager and Proteus both have them.

Mike,

In my original post "CNS" stands for converged network services which is essentially structured wiring system with dedicated switches and routers for use of various IP network systems such as data/voice, BMS, electronic security, lighting control etc. Having thought about this some more I think best approach is risk assessment omit RCD and retain AFDD on 32A SPN commando socket on basis that AFDD nuisance tripping is not likely. 

Mike,

In my original post "CNS" stands for converged network services which is essentially structured wiring system with dedicated switches and routers for use of various IP network systems such as data/voice, BMS, electronic security, lighting control etc. Having thought about this some more I think best approach is risk assessment omit RCD and retain AFDD on 32A SPN commando socket on basis that AFDD nuisance tripping is not likely. 

If it is for CNS then it would be prudent to install a UPS (Uninterruptible Power Supply) especially as the NG (National Grid) has come close in recent months to brown outs

Sergio, 

Agreed. Local UPS will be provided within the cabinets. 

would be prudent to install a UPS (Uninterruptible Power Supply)

Not a bad idea, but it does have limitations - typically a UPS will only support things for a few hours at most . If the installation is unattended then the nett effect of the UPS on a protective device trip could be no more helpful than to delay loss of service for a short time. That could be mitigated if there was some remote monitoring and people on call 24x7 to attend - but that sort of thing can be costly too.

   - Andy.