reduced cable

Firstly can we confirm you mean 3 phase at 400V phase to phase I presume you probably do, as from your figures (at 30A *3 *230 is ~ 21000) 

So is your concern that the 100 amp protection is a bit coarse for the cable, or that the motor may catch fire if overloaded with nothing tripping, or that the Zs will be too high ?

It  rather depends how serious it would be if say the cable or motor got cooked and how the starter handles overloads.

Mike.

Why would you do any of these things? Really you have asked the wrong question, presumably, you have no understanding of circuit protection. The motor overload protects the whole circuit, as does the CPD. The sustained circuit current is set by the overload to a satisfactory figure for the cables you have described. The question is "would the 100A CPD open before the 4mm cables under short circuit conditions?". The answer is to use the adiabatic formula and you will find out if it is satisfactory. The circuit PSCC is important as if (it almost certainly isn't) it is quite low the 100A fuse may go outside the expected fusing time and cause the cable to overheat seriously outside the adiabatic time limit. This is probably not particularly important as it is a temporary arrangement. The soft starter fuses are probably 63A and will probably fail first anyway. There is no reason at all to suggest that the overload may not open under overload conditions, they are very reliable and the most likely fault would be that the soft starter does not disconnect the load.


The circuit is therefore protected against short circuits, sustained overload, and motor faults in a satisfactory way. If you fit 40A fuses it is possible that you will get a failure, the loss of a phase, and possible motor damage. This would be stupid and expensive, and more importantly completely unnecessary.

is there an issue just relying on the overload

Not really - it's common in industrial motor circuits for the circuit protection to provide only fault protection and thus to be rated significantly higher than the cables they protect. That way when the motor overloads (commonly due to mechanical overload) it's the local overload that trips rather than the remote circuit protection. In the days of fuses there were even specific "motor" fuses that had a higher rating than their size would normally indicate - so in effect you could put something like a 30A fuse in a 20A holder.

   - Andy.

So using an overload to protect the cable is fine. I understand circuit protection David sorry if I offended you in anyway i just wanted clarification if we could rely on protecting the cable through an overload relay as I’ve been told before they are purely just to protect the motor hence me asking if the addition of fuses are needed. I take it fuses would only be needed if the disconnection times cannot be met?

I take it fuses would only be needed if the disconnection times cannot be met?

Yes, you still need to disconnect both to protect the cables in the case of a fault and for shock protection. The adiabatic is your friend for that (or comparing energy let-though with k²S² if the upstream protective device is a circuit breaker and you're looking at magnetic operation).

   - Andy.

Uh-huh.

a 100a MCB will not protect a 6mm cable from heat death, if you can trick the load into drawing more than about 50A but less than 100. But, the motor overload relay will respond  essentially like the slow thermal part of an MCB and if it is set to something in the range  40-50A the motor will run and that long duration overload will be impossible, and the cable will last for the duration. (*)

The second question is now only if the cable gets spiked (an accidental damage sort of short circuit) between the 100A MCCB and the overload relay, is the current enough to get the 100A trip to do its duty before the cable melts, and that is a Zs/pssc trip time question, where you need data on that 100A breaker.

Assuming a TN supply, and that you do not have a voltage drop problem at full load, then that prompt clearing case will also be met.

If the Ze is high enough that the 100A may not fire instantly under all conditions,  then it may need more thought.


Mike


*And note that  a cable routed in a way that is rated at XX amps does not mean it will burst into flames if it passes XX + 10% all day, nor if it passes twice XX for a few minutes and then has time to cool off. If it happens a lot, it may become  de-plasticised and stiff, and be so much expensive scrap when you wind it in, but it won't fail in an immediate and dangerous way - though it may get painfully hot to the touch.


It is quite fun to look at the automotive ratings for cables, compared to BS 7671 as they are based on a much hotter, and rather shorter, life  and see that your 1mm2 cable is now good for the  best part of 25A...