The upstream BS88 will be enough for the fault current protection for the installation. It would be better to get a 7kA rated breaker, but, in the worst case, the breaker will be damaged, but, would it really be the worst case that does happen?

There is added resistance from the source to the end of the final circuit. To get that fault current (and how has it been tested, MFT's are not reliable at higher currents) the fault would have to be at the source. On your final circuit, the impedance will reduce the fault current somewhat, and may even comply at the far end. 

If it bothers you that much, and is likely to cause more than an inconvenience, an easy solution is to add a fuse (BS88 - 16kA+) in an enclosure,and run the final circuit through that, but I think that is going too far, your upstream BS88 is enough for most people.

Hi Alan, thank you for your response.

We probably have at least 50 of these installations on site, so I need to either say we replace for breakers with appropriate ratings or prove why "The upstream BS88 will be enough for the fault current protection for the installation." 

How can I prove this?

Hi Alan, thank you for your response.

We probably have at least 50 of these installations on site, so I need to either say we replace for breakers with appropriate ratings or prove why "The upstream BS88 will be enough for the fault current protection for the installation." 

How can I prove this?

I am surprised that what sounds like an industrial installation has MCBs with such a low breaking capacity, but perhaps they are old.

You need to ensure that the PFC is high enough at the extremity of a circuit to trip in the event of a fault there, but low enough to ensure that the protective devices are not damaged by a fault at the supply end.

It may be that the let through energy of the fuse will not be high enough to damage the MCBs, but I shall leave it to those cleverer than I to discuss that.

In the mean time, it sounds as if the installation does not comply with 432. Note that if the MCBs were changed for 10 kA ones, and if the CPCs have been selected by calculation, you would need to ensure that they still comply because I²t is likely to be higher.

I am surprised that what sounds like an industrial installation has MCBs with such a low breaking capacity, but perhaps they are old.

You need to ensure that the PFC is high enough at the extremity of a circuit to trip in the event of a fault there, but low enough to ensure that the protective devices are not damaged by a fault at the supply end.

It may be that the let through energy of the fuse will not be high enough to damage the MCBs, but I shall leave it to those cleverer than I to discuss that.

In the mean time, it sounds as if the installation does not comply with 432. Note that if the MCBs were changed for 10 kA ones, and if the CPCs have been selected by calculation, you would need to ensure that they still comply because I²t is likely to be higher.