Adiabatic question

Paul_K · Feb 16, 2014

Hi guys,

I have brain freeze and need some help!

I'm currently looking to run in a radial socket outlet circuit to a bedroom from a spare way in the consumer unit. The total circuit length will be approx 10 meters of 2.5 t&e. The supply is a TNC-S arrangement with a Ze of 0.13ohm (by direct measurement). The circuit protection is by way of a 20amp Type B BSEN60898 mini circuit breaker. NO RCD!!

Normally I would just follow the typical values in the OSG. This time though, I decided to just calculate it... Have to keep doing it once in a while or I'll forget!! :-/
The cable itself is fine (obviously) until I check the cpc size using the adiabatic equation. Can someone tell me if I've got this right? :-/

Ze = 0.13

R1+ R2 = 0.23 (adjusted for temperature @ 10 x 19.51 x 1.2)

Zs = Ze + R1 + R2
= 0.36

230/0.36
= 639A

K factor = 115
Assuming instantaneous time of 0.1sec

S = sroot(639*x0.1)/K
= 1.76??

Thanks in advance!

silverfox1 · Feb 16, 2014

Hi Paul as your figure calculated using the adiabatic equation will not get a corresponding figure of t from the time / current graph you can obtain the I2t energy let through figure from your MCB manufacturer, you can then substitute it into the adiabatic equation.
The I2t figure for a crabtree starbreaker 20A type B 6K mcb is 22260 put that figure into the adiabatic .
Square root of 22260 / 115 will give minimum size cpc of 1.3mm.

Guest569 · Feb 16, 2014

Just get it done....seriously.

HandySparks · Feb 16, 2014

silverfox1 said:
Hi Paul as your figure calculated using the adiabatic equation will not get a corresponding figure of t from the time / current graph you can obtain the I2t energy let through figure from your MCB manufacturer, you can then substitute it into the adiabatic equation.
The I2t figure for a crabtree starbreaker 20A type B 6K mcb is 22260 put that figure into the adiabatic .
Square root of 22260 / 115 will give minimum size cpc of 1.3mm.

Thanks for reminding me of this; they list I²t figures in the technical section at the end of their catalogue. As it happens, I generally use Crabtree Starbreaker, but does anyone know whether these figures are typical for other makes of BS 60898 circuit breakers?

silverfox1 · Feb 16, 2014

HandySparks

there is a reference table for minimum size cpc when using energy limiting class 3 mcb in onsite guide, table B7 page 120.

Paul_K · Feb 16, 2014

I hate this forum!!

One of the reasons I never use it, is there's far too many egotistical smart arses on it who are only here to measure the length on their dicks with each other!

Thanks Silverfox1 for the feedback, it's much appreciated! At least there's someone willing to take the time to give a sensible answer!...........

Now to the admin,

Please delete my account, your forum is a disgrace, and seems only to serve very little purpose other than to serve up illiterate people trying their very best to sound clever!

Tedious and off putting!!

GLENNSPARK · Feb 16, 2014

Paul_K said:
I hate this forum!!

One of the reasons I never use it, is there's far too many egotistical smart arses on it who are only here to measure the length on their dicks with each other!

Thanks Silverfox1 for the feedback, it's much appreciated! At least there's someone willing to take the time to give a sensible answer!...........

Now to the admin,

Please delete my account, your forum is a disgrace, and seems only to serve very little purpose other than to serve up illiterate people trying their very best to sound clever!

Tedious and off putting!!