Fusing Factors

[m4tty]

Fusing Factors

BS3036 - 1.8 to 2.0

BS1361 - 1.6 to 1.9

BS88 HBC - 1.25 to 1.27

MCB's up to 1.5

Then Table 7.5 OSG P58 states

B Type 3-5x In

C Type 5-10x In

D Type 10-20x In

I understand that if youve got switching surge flouescent lighting C type is favouable over B type if Zs complies as it may sometimes trip a B Type and not a C Type but im getting confused as I read MCB's fusing factor is 1.45 or is it different depending on what Type B/C/D it is.

Im confused but not very good at explaining things. If anyone gets what im saying and can explain it would be appreciated

Thanks in advance

Matty

 

[Brian 157]

The fusing factor is different for each type of MCB, higher the letter, higher the fusing factor, this is why we can get away with higher Zs values for lower letter MCBs. If you look at the time/current graphs in BS7671:2008, Appendix 3 you will see that type Bs require up to five times the rated current to trip in time (ie 6A MCB needs 30A), type Cs up to ten times and type Ds up to twenty times. The dodgy ones to watch out for are the old BS3871 type 4s, these had a fusing factor of up to 50 & are not the same as type Ds (a common mistake).

 

[seajayess]

Hi the 1.5 is what it will handle for a long time before tripping.

The B X 5 C x 10 D x 20 on MCB is what current it takes to trip instantly under 0.1 of a second. These x 5 x 10 x 20 results are what are used to calculate your Zs value as in an event of a fault this would be the greatest value of resistance you should have to allow the MCB to trip.

Try it out 32 Amp B type MCB

32 x 5 = 160 Amps

230 Div 160 = 1.43 ohms

Now x 0.8 to allow for cable temp

1.43 x 0.8 = 1.15 ohms

Look in BRB you will see 32 Amp B MCB is 1.16 ohms

hope this helps CJS

 

[Nicky Tesla]

Hi the 1.5 is what it will handle for a long time before tripping. The B X 5 C x 10 D x 20 on MCB is what current it takes to trip instantly under 0.1 of a second. These x 5 x 10 x 20 results are what are used to calculate your Zs value as in an event of a fault this would be the greatest value of resistance you should have to allow the MCB to trip.

Try it out 32 Amp B type MCB

32 x 5 = 160 Amps

230 Div 160 = 1.43 ohms

Now x 0.8 to allow for cable temp

1.43 x 0.8 = 1.15 ohms

Look in BRB you will see 32 Amp B MCB is 1.16 ohms

hope this helps CJS

zackly wot i woz gunna say