11kV/400V TX with no LV protection

This might be acceptable.

Short conductors connecting a transformer to fuses or MCCBs are permitted without any short circuit or overload protection.

Also overload protection can be omitted if the characteristics of the load prevent overload. For example if the cable is good for 300 amps, but the load can not exceed 300 amps, then that is probably acceptable. Short circuit protection would still be required, but that MIGHT be provided by the fuses or circuit breaker on the 11 kv input to the transformer.

Thanks for the reply. I understand that overload protection might not be required if there is no chance of overload but where is it stated that short circuit protection is not required if connecting short length of cable and what constitutes as short length.

It is somewhere in the regulations, words to the effect of "conductors that connect generators, batteries, or transformers with control panels do not require overcurrent protection"

You need to realise that protection on the HV side of the transformer is much easier than the LV side, and that transformers are extremely robust when it comes to overload, particularly in larger sizes, or with Oil cooling. Protection of the cables on the outgoing side is also very difficult, where there will usually be several cables in parallel and faults are extremely unlikely. Your transformer HV side presumably has either fuses or circuit breakers of perhaps 60A rating, whereas the secondary may need 1500A fuses. In fact the primary fuses give significantly closer protection to the TX without any difficulty over fault and loop impedance, which may not be as low as you understand from BS7671 and the multiple parallel cables probably will not limit fault currents sufficiently anyway, so the fuse may never fail, just a big burn-up!

I concur with respect to over load but what about short circuit protection. My argument is, if the cable between the outgoing side of the TX and the building it is servign (over 30m away) was damaged, what is providing the protection to that cable? The busbar in the Transformer. Surely the outgoing side of the Tx needs protection?

An output short circuit WILL blow something, and the best thing is the HV protection.  If not this then one of the cables may blow open circcuit but this may not remove the short. You are talking fault powers of many 10's if not 100s of MWA here for a 1MVA TX.  You probably shouldn't be anywhere near this, because you have a complete misconception of "Fault Protection". What are you attempting to protect, because it certainly isn't the TX or the cables? This is exactly why we have HV circuit breakers and fuses at each transformer, you are thinking BS7671 all the way, which at higher powers simply doesn't work.

It does merit consideration. You also need to consider faults at the receiving switchboard upstream of the first protective device.

For a private transformer or similar Interphase fault protection should be achieved by coordinating with the HV protection (fuses/relay). As others have suggested this is normally fine - but someone should check the clearance times and cable/busbar rating.

For a long cable it is possible that the fault impedance will reduce the prospective to the no-trip region of the HV fuse / relay setting, in which case you might indeed need something on the LV spill box

Similar applies for earth faults but if HV protection isn’t likely to operate in a timely manner restricted earth fault schemes will ensure rapid clearance on high impedance faults.

I assume that your transformer isn’t big enough to justify differential protection!

Mitigating the risk by design such that faults are unlikely is a wise strategy regardless.

David,

I am unsure why you don't think BS 7671 is not relevant here. I am talking about the output side of a transformer which is 400V and completely relevant under BS 7671

It is not relevant because this protection is not covered. BS7671 basically covers the LV side of installations, generally supplied by a DNO. Your private transformer puts you in the position of a DNO, and therefore you should at least examine their practice. A transformer is simply a device to change voltage, and the power on each side is directly related. As I have pointed out secondary currents are completely reflected in the primary, and this is the best and easiest place to provide protection against both overload and short circuits. The whole supply chain is yours, up until the HV metering. The BS7671 definitions of protection are therefore simply reflected to the HV supply, and there is no reason at all to not use this feature. The huge fault power available direct from the Grid is your main consideration, not the secondary cables, which you will find are very difficult to design with any normal protection anyway. Assuming the whole TX supplies one DB, the connecting cables are the least of your problems, the PSCC probably being the biggest. You will find it is probably 150-300 kA, and the weakest link is those connecting cables. HV fusing or CBs provides much the cheapest protection and anyway is required for your Grid connection.

To re-assure yourself, you only need to check, what is the upstream protection for the TX.

Given the TX itself will add an impedance of a couple of percent, it is safe to assume upto something between 20 to 50 times the nameplate rating,  that transformer action will be linear, and every  kilo-amp on the LV side at 230V reflects back to be 230/6400 of a kA, or about 35 amps on the primary. Depending on the design, there may be  expulsion fuses or current transformers and a more complex time integral electromagnetic trip, maybe even a boiling oil pressure operated  "Buchollz" trip as well, but there will always be something.

A really bad fault on the LV side of a magnitude that  might damage the transformer will simply cut the HT side off, and that includes shorted windings in the transformer itself, which no external LV fuse would guard against.

Mike