Main and sub distribution boards - Circuit Breakers

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.

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.