MCB tripping time equation?

[CRAIGNEWHAM]

Evening,

Can someone tell me what the equation is to work out how quick a CB will trip from the Amps its getting?
I know there are tables in 7671, but I'm sure there is a formula where you can work it out from the fault current amps etc?

Thanks in advance
Craig

 

[tony mc]

You would have to know how many amps the fault was and then look at the times curves at the back of 7671 for that protective device to get the time.

You can work out the ZS for the mcb on 60898 type B C or D.

 

[CRAIGNEWHAM]

I thought I could remember there being an equation where if you knew what amps a fault would have, then you could work out the time it would take to trip it?

 

[i=p/u]

lighting circuit 5A tybe b cb has maximum measured efli of 7.42 ohms and this canbe achieved on tncs system if no rcd and cable run is less than 88m.. correct me if im wrong, doubt it lol

 

[i=p/u]

tybe b is 5 times c 10 times d 20 times

 

[telectrix]

yes, to trip within the time stipulated. what the op mis asking is calculating the trip time for differenf values of faULT CURRENT. SO TIME/CURRENT CURVES.

damn it, need bigger keys to suit my sausage fingers

 

[CRAIGNEWHAM]

So is there no equation to work out the trip time from any current?
Thought I did one when I was on my course?
Been looking thorugh 7671 all afternoon!!

 

[i=p/u]

ive seen a time curve graph i think on this and was awkward to read

 

[telectrix]

back of brb.

 

[i=p/u]

waiting for bgb

 

[CRAIGNEWHAM]

The graph is ok, but I wanted to find the equation?
Or am I wrong and there isnt one?

 

[tony mc]

Is this what you were working out

Tabulated ZS is what is in 7671 but we have to correct that taking into considaration ambieant temp and conductor temp under fault.

You would of been calculating the ZS Type b x 5 c x 10 d x 20

Thats why we use the rule of thumb method.

Type B 6A 60898 MCB 5 X 6 = 30 230 /30 = 7.6666 7.666 X 0.8 = 6.13 ohms

So if your measured ZS is 6.14 ohms or less the protective device will operate within the safe time limits TN 0.4 sec TT 0.2 Sec for ccts 32 A or under.

It actually trips alot Quicker if you look at the time curves but you are fulfilling the diconnection time if the above is met.

Is that the sort of thing you were doing ???

 

[CRAIGNEWHAM]

Well I'm ok with tha graph etc, I just thought there was an equation where if you had a fault current you could work out how quick it would trip?
I just want to find it for my peace of mind! been looking for hours, but if there isnt one then my mistake!

 

[tony mc]

Sorry I don't know of one as I use the time curves in the regs.

 

[CRAIGNEWHAM]

Ok, cheers anyway