Circuit protective devices, MCB types?
- Thread starter Mike2007
- Start date
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afaict there is no type A or at least if there is its not availible from any normal supplier.
the types tell you where the fast trip point is set. iirc for a B its 3-5 times rated for a C its 5-10 times rated and for a D its 10-20 times rated.
Basically the higher the fast trip point the lower you have to get the fault loop impedances but they are also less prone to neusense tripping.
Unless you have a full understanding of earth fault loop issues steer clear of all type D breakers and from type C breakers above 16A.
the types tell you where the fast trip point is set. iirc for a B its 3-5 times rated for a C its 5-10 times rated and for a D its 10-20 times rated.
Basically the higher the fast trip point the lower you have to get the fault loop impedances but they are also less prone to neusense tripping.
Unless you have a full understanding of earth fault loop issues steer clear of all type D breakers and from type C breakers above 16A.
Are you saying the C's and D's trip faster, thus the loop imeadance test Ze or Zs is hard to do because they trip? I thought C's were sometimes used when there might be a surge, as in having a welder in use?
I have B's in my own consumer unit across the board.
No, you need to overload C/D types more before they will trip so the EFLI needs to be lower to ensure disconnection within a suitable time.
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no the fast trip point is higher so it takes more current to make them fast trip.
MCBs have two seperate trip mechanisms. One is a thermal trip mechanism, it starts to act at just above rated but it trips very slowly at such currents as the current gets higher it gets faster but still not very fast. This trip mechanism exists to deal with overloading.
The other is a magnetic trip mechanism. This doesn't trigger until much higher currents but when it does trigger it trips extremely fast (iirc the standards require 100ms but some are built to trip in less than one cycle). This trip mechanism exists to deal with short circuits.
If the fast trip point is set at too low a current surges from equipment can cause unwanted tripping (bad). If it is set too high then a short circuit may not trip the breaker quickly (very bad).
To determine if a MCB will trip on short to earth the Zs (P-E loop) at the furthest point on the circuit is either measured or calculated (from Ze and r1+2). This can then be used to calculate if the current will be sufficiant to fast trip the breaker. Really it would be a good idea to do the same for the PN loop but generally noone bothers (the assumption is that the P-N loop will generally be lower than the P-E loop)
In other words using a type B is always the safeest choice but may be prone to unwanted tripping. Using a higher type may require thicker cables or not be able to be made safe at all depending on the conditions in the particular installation but may avoid unwanted tripping.
MCBs have two seperate trip mechanisms. One is a thermal trip mechanism, it starts to act at just above rated but it trips very slowly at such currents as the current gets higher it gets faster but still not very fast. This trip mechanism exists to deal with overloading.
The other is a magnetic trip mechanism. This doesn't trigger until much higher currents but when it does trigger it trips extremely fast (iirc the standards require 100ms but some are built to trip in less than one cycle). This trip mechanism exists to deal with short circuits.
If the fast trip point is set at too low a current surges from equipment can cause unwanted tripping (bad). If it is set too high then a short circuit may not trip the breaker quickly (very bad).
To determine if a MCB will trip on short to earth the Zs (P-E loop) at the furthest point on the circuit is either measured or calculated (from Ze and r1+2). This can then be used to calculate if the current will be sufficiant to fast trip the breaker. Really it would be a good idea to do the same for the PN loop but generally noone bothers (the assumption is that the P-N loop will generally be lower than the P-E loop)
In other words using a type B is always the safeest choice but may be prone to unwanted tripping. Using a higher type may require thicker cables or not be able to be made safe at all depending on the conditions in the particular installation but may avoid unwanted tripping.
I found a 63A type D MCB feeding a refrigeration plant from a submains DB on a periodic yesterday. The circuit was 0.01 ohm below the maximum permitted Zs 
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