Type B MCB -v- Type C MCB tripping

[Cremeegg]

Would you generally expect a new 2000W Bosch anglegrinder running 230mm discs to trip a new 16A type B MCB on starting up?? It's not tripping an older MK type 1 20A. Grinder appears fine. Just seeking thoughts before I check Zs and consult the big brown book to see if I can change to Type C MCB.

 

[Taylortwocities]

Tom Jones (its not unusual). You need to ensure EFLI is good enough before swapping the CB for a C curve.

 

[plugwash]

IIRC a type 1 is a bit more sensitive than a type B of the same rating but your type 1 is of a higher rating.

remember there is quite a wide range of fast trip points allowed for a breaker of a given rating and type so what trips one breaker of a particular type may well not trip another.

I would go for a type C if circuit characteristics permit.

 

[Cremeegg]

Thanks for that – you’ll see that I’d already reminded myself to check before making a trip to get a Type C. The breakers are MK - clearly they comply with the curves in the BS but are they particularly sensitive?

Now the rugby’s finished I’ve got my brown books off the shelf and its now table confusion time. Big brown book Zs figures for Type B 16A are 3.0 whereas little brown book figures are 2.4.

OSG App 2 p89 explains that OSG figures are lower than BS7671 as OSG are taken for assumed 10C conductor temperature whereas BS7671 are designed for “normal operating temperature”.

How do we define that – just depending on the season in an external situation it could be anywhere from below 0C to over 20C (maybe not this summer!!).

Applying the incremental correction factors in Table 2E OSG it looks like the “normal operating temperature” is taken to be 70C. This is stated to be the “maximum operating temperature” earlier in OSG p88.

Anyone care to explain?

Have just found the original Zs figures and will check the Zs tomorrow (just in case it’s changed for the better since the install) but I suspect I’ll be re-running circuit and increasing the cable CSA to 4mm to get it down to within limits for a Type C.

Thanks in anticipation

 

[plugwash]

My understanding is the figures in the regs book are for the actual resistance at the time of the fault which means worst case the cable may be at 70C, the figures in the OSG are those you will measure when testing the circuit at normal room teperature.

 

[JohnD]

the MCB in my hand is marked "40º C"

 

[Spark123]

Could that be a calibration figure for the MCB?

 

[JohnD]

I assumed it meant that is the temperature at which it performs to spec.

(is a memshield 2)

 

[Spark123]

Yeah, thats what I'd think it is, the spec of the MCB.
Plugwash's post is correct in what he is saying. The cable resistance changes with temperature hence the maximum efli needs to be adjusted from the tabulated value (240/5xMCB rating for a B type, 240/10xrating for a C type) where the conductor resistance for a 70ºC cable is at its highest to a temperature which is comparable to it being measured. This is done using the correction factor 1+(0.004xtemp), hence for a drop in temperature of 60ºC the max efli figure needs to be divided by 1.24 (or multiply by its reciprocal i.e. 0.8 ) to give the maximum measured efli for a conductor at 10ºC.

 

[Adam_151]

For those of us too lazy to work it out, we just apply the 3/4 rule of thumb, as long as the EFLI isn't greater than 75% of the value from big brown book then it passes, so as long as its less than 0.75x3=2.25 then it passes, off the top of my head, the small brown book does a similar thing but for some god forsaken reason uses 80% instead of the widely accepted 75%

 

[Spark123]

the small brown book does a similar thing but for some god forsaken reason uses 80%

or multiply by its reciprocal i.e. 0.8 ....