Main and sub distribution boards - Circuit Breakers

I hope  I have this right,  given the lack of schematic capture on this forum is this about it ?

 If so, if there is a fault on cct Q2, then it is a toss up if Q1 or Q2 will fire or both, if neither side has programmable delay. 

Even if one had a higher rating than the other, say Q1 was 50A, for a high current fault they will still race. 

What loads are present, rather than what breakers, will influence how this behaves and if you can fiddle the ratings a bit..  Knowledge of the loads - will they all be on at once for example, affects the ratio of outgoing circuits to incoming cable capacity.  Many industrial processes have things that are never on together, and we can rely on that knowledge to avoid over sizing the supply.

 Do we care if the ‘many MCB 2’ go off it there is a fault on the Q2 circuit ?

For the other question, in general on TT 3 phase systems you break the neutral, but otherwise 3 pole breaking is preferred.

Really more info is needed to put this in context before we solve the wrong problem.

Also do you really mean 230V 3 phase in the sense of 230V live to live, or 230V live to neutral and 400v live to live. The latter is the UK and EU norm.

Mike.

I was about to respond to this when Mike's response popped up. However I will still add to what he has said.

Regarding 1, I am not sure what you mean by ‘check manually’. The way to check is to look at the current-time characteristics of the two breakers and see if there is an overlap (where an overlap indicates lack of discrimination). As the discrimination is achieved either gestcurrent or time (or both) the fact that both are 25A breakers and both MCBs (and hence unlikely to have adjustments) suggests to me that you do not have any discrimination between the two (and if they are identical breakers you can be certain of this). It is possible to get current injection equipment to test this but unlikely to tell you anything you can't see from the current-time graphs. This does ignore the fact that the upstream breaker will have a higher current than the downstream breaker by virtue of the other circuits in operation, but at fault current levels this will probably not make that much difference.

Regarding 2, replacing the breaker with a lower rating CB is an option, but only if you have the option of doing so. If you have a 20A load then putting in a 10A or 16A breaker is not practical. Also don't forget that if you have a motor the starting current shouldn't be tripping the breaker so a 16A flc motor may need a larger breaker rating than 16A.

I am not aware of any standard which limits the number of CBs on a secondary distribution board. It is normally down to how many circuits are needed and the size of the board (that is, how large you can make it to fit in the space available) as you can only fit in as many CBs as there is space available, which may drive you to multiple DBs.

For the rest, I will bow to the wisdom of mapj1….

Alasdair

Thank you Mapj1!

What is the thumb rule for the discrimination between the upstream breaker and the downstream breaker? by thumb rule I meant, what should be the minimum multiplier to the downstream breaker to arrive at the upstream breaker for proper coordination? 
for example, the upstream breaker should be twice or thrice the breaker rating of the downstream breaker to get at least partial discrimination?

I read that here are certain cases where you may want the same breaker at both ends  (Upstream and downstream), but what are those cases?

Say, if I want to increase the downstream breaker rating, for discrimination purposes I should increase the rating of the Upstream breaker in which case for a fault current at the feeder, the upstream breaker (if it is type C) may not act in time and the cable becomes fuse. I wanted to avoid this

Why neutral has to be breaked in TT systems?

Yes the systems are 230V AC, Line-Line, 3 Phase output from the UPS

What is the thumb rule for the discrimination between the upstream breaker and the downstream breaker? by thumb rule I meant, what should be the minimum multiplier to the downstream breaker to arrive at the upstream breaker for proper coordination?

Unfortunately it's not that simple. With fuses you usually get decent discrimination with a 2x factor between fuses (or sometimes less - often 1.6x if both fuses are to the same standard). With MCBs though there's no natural in-built delay, so a large fault current will very likely cause both MCBs to de-latch before either has fully opened. Your only chance is to arrange things so that a fault downstream of the second MCB would be too small to operate the first MCB - but that rather goes against the principles of ADS/fault protection where a fault just before the 2nd MCB needs to reliably open the first MCB, and since the 2nd MCB doesn't add significant impedance to the circuit, a fault just after the 2nd MCB is likely to produce a very similar fault current and so still risk opening the first MCB (as well as the 2nd one).

MCCBs sometimes employ a few tricks to get discrimination - e.g. in-built delays or even a system where the contacts might start to open slightly but immediately close again if the fault goes away, but such things aren't seen in simple MCBs.

Why neutral has to be breaked in TT systems?

It's a basic isolation principle - N can be left connected where it's reliably at a very similar potential to the Earthing system - which is pretty much guaranteed in a TN system (the short periods during faults notwithstanding).  On a TT system however, there's no metallic connection between N and PE of the installation, so significant voltage differences can develop - possibly the nastiest example is if a (non RCD protected) L conductor comes into contact with the general mass of the earth (could be anywhere on the LV system, not necessarily within your installation) - which can skew the earth reference of the entire supply system, taking the supply N tens if not hundreds of volts away from true Earth.  Typical causes are either an overhead line coming down or a L-PE fault in another TT installation with a faulty (or missing) RCD.  The same thing happens with TN customers on the same supply of course, but their PE connections follow N to whatever potential it happens to be at, so the risks (inside buildings at least) are much reduced.

Presumably the arrangement is TN downstream of the UPS though.

   - Andy.

Andy beat me to it really - if you really must have reliable discrimination between breakers for all fault levels, and not just discriminate for modest overloads, you have to have programmable delays for all but the ones nearest the load.  

Fault levels above the threshold for the magnetic ‘instant trip’ part of the larger circuit breaker do not give any sort of reliable discrimination. 

Actually usually this is OK in practice as zero ohm faults are quite rare, and with care about what shares the sub-main, the worst of the side effects can be designed out. And at the back, for really critical things we can decide to fit “death or glory”  fuses that only operate when the end of the world is nigh, as it were, rather than magnetic breakers. Chosen well this also relaxes the blast containment aspects and may allow for lighter switchgear, as the equivalent I2t after the fuse can be a lot less than it is in front of it.

If you must have delay, then you need MCCBs and the curves in this paper may help clarify how things are done much  better than I can in words.

Using a delay into any fault level with no upper bound “instant trip” current is scary and something to think very hard about, as even a fraction of a second feeding kA into a dead short is a lot of joules per ohm or I2t, and a lot of potential damage to cables / arcflash etc. Imagine waiting a third of a second for things to strat to think about  cut-off, it could be like a bomb going  off. Sometimes the delays at the top of a cascade can be as much as a few seconds, but it is not nice if it is ever called upon to perform its duty.

I'm not really joking  -  at the indoor substation level you may find that one wall  is deliberately weak, rather like the ‘tear here’ lines on paper forms, to give a controlled failure mode if the worst happens, and not to damage more important structure  Saving the equipment is not a consideration then.

If you can, try to design the requirement out, or at least only guarantee to fully discriminate for faults up to a certain maximum current, then say above that all bets are off.

Mike.

AJJewsbury: 
Your only chance is to arrange things so that a fault downstream of the second MCB would be too small to operate the first MCB - but that rather goes against the principles of ADS/fault protection where a fault just before the 2nd MCB needs to reliably open the first MCB, and since the 2nd MCB doesn't add significant impedance to the circuit, a fault just after the 2nd MCB is likely to produce a very similar fault current and so still risk opening the first MCB (as well as the 2nd one).

I think that you have to get lucky. I think that I may just get some discrimination between a 63 A MCB in the DB at the intake and a 32 A one in the house DB by virtue of the Ze being sufficiently high and the cable being long enough. As Andy says, the PFC at the load end of the distribution circuit needs to be high enough to give instant trip, but by the time that we get to the sockets, the PFC should be below 315 A.

In practical terms, any fault is much more likely to occur in an appliance than in the fixed wiring, at least in my house.

The first thing you need to decide is whether you need to have discrimination and, if it is only a “nice to have”, how much are you prepared to pay for it.

Yes, 2 to 1 is a good rule of thumb but it is not guaranteed to give you discrimination and with a breaker that reacts quickly to an earth fault, seldom does.

Faults tend to happen in the final user equipment so discrimination here is useful. If you have a gentle overload then any difference in rating should give discrimination. If you have a fault in a main feeder then maybe it does not matter if everything goes off, better that than an even bigger bang.