another calc query for the experts

[ezekiel]

sorry to be a pain:

measured zs on lighting circuit of 0.95ohms
max zs for 6a breaker in regs is = 7.67ohms

do these figures sound correct (only on a test board so no long runs etc)

then divided volt by zs for a fault current = came up with 242?

not sure how this is compared to charachteristic graphs for disco time

any help appreciated

 

[Spark123]

Yes, the figures sound correct however the figure from the regs is the maximum allowable efli which is when the cable is at its maximum normal operating temperature.
In order to compare a measured efli you need to convert the BS7671 tabulated figure. As a rule of thumb, the Maximum measured efli should not exceed 3/4 (three quarters) of the maximum tabulated.
There are some tables in the back of GN3 (2008 version) which give you more accurate maximum measured efli for various protective devices for a given cable type and a formula for working it out, iirc for a 70°C cable measured at 10°C is tabulated efli divided by 1.24.

 

[ezekiel]

thats great (yet again spark)

but how is this compared to time/current charachteristic charts?

ie:

notes from practical guide book:
once zs established divide by voltage for fault current. apply this calc to time/current graph to determine actual disco time for circuit.

60898 6a rating has current of 30a for 0.1 - 5 secs

my calc gave a fault current of 242 and this is confusing me , cant see how it relates to charts

 

[Spark123]

It means that it will go in 0.1s or less which is good when using an MCB. MCBs are a little bit different to fuses as in once the magnetic side operates it trips 0.1s or less, this magnetic part shown as the straight line on the graph. This differs in position for differing MCB types.
The thermal part of the MCB is shown as the curve - this is the same for all MCBs of the same current rating (but will cut off at a different position as the magnetic part takes over.)
You may be better trying to understand fuses first, these have a more understandable relationship between the current and time.

I = V/Z

 

[ezekiel]

you fixed it again

took a while but started with fuses as sugested and can see where its leading now -

 

[GaryMo]

Look at the relationship between current and time. Higher the fault current, lower the disconnection time. To ensure fault current will be as high as possible (to disconnect as quick as possible) we construct circuits to have low resistance, hence maximum Zs figures we work to.

Have a read of appendix 14 which details measurement of earth fault loop impedance and the consideration given to the increase of resistance of conductors with increase in temperature under under conditions.
Also, as Spark123 suggested, page 112/113 GN3 (17th).

 

[ezekiel]

dont think i would of survived through college as long - learn more through this forum than from lecturers - slowly and surely getting there though - many thnxs