Calculating Max EFLI for different breakers

[Jim2287]

If i have a Semi enclosed fuse of 30Amp for example, how do I calculate the maximum earth fault loop impedance that would allow the breaker to operate in the specified time 5 seconds or 0.4 of a second ?

Looking at the Regs they give you tables and graphs,
but they also say these values only apply if the circuit is at a given temperature (what temperature is that?)

Is there a correction factor somewhere available in the regs? so that you can use their values and make them relevant to your scenario

any help would be appreciated

 

[didthathurt]

This is all in App 2 of the OSG. Table 2E, p93 of the Brown OSG, gives temp corr factors.

 

[Jim2287]

Thanks very much didnt look in OSG which i suppose i should have looked there first

thanks

 

[Spark123]

Look at the graph for the given type of protective device, this will give you the current required for the device to operate in the given time. 240/Current = max efli. This is the max efli for the cable at its operating temp, i.e could be 70deg C. To get the maximum measured efli at 20deg C there is a 50deg C drop hence using the temperature coefficient of copper which is 0.004 so therefore 50 x 0.004 = 0.2 so the maximum measured efli will be 20% less at 20% C. Therefore Max EFLI x 0.8 = Max measured EFLI for a 70deg C conductor measured at 20deg C.
For circuit breakers the required current to trip is 5x rating for B type, 10x for C type and 20x for D type.

 

[Taylortwocities]

If I remember the figures in the brown book are for 70º C and the OSG is at 20º C, or is it the other way round?

 

[Spark123]

You are sort of correct, the BS7671 figures are operating temp figures which will depend on the temp rating of the cable. The osg quotes figures for a 70deg C cable measured at 20 deg C.

p.s. how do you do the little degrees sign?

 

[Taylortwocities]

put your finger on the alt key and hold it there
type 0186 on the numeric pad release yr finger and Robert is your mother's brother

also
alt 0178 = ²
alt 0181 = µ
and there's more (come here)

if you go to character map on the windows help and select a symbol, the ALT #### sequence is shown at the bottom

 

[Jim2287]

I thought the correction factor would be so you can design circuits for a operating temperature, because resistance increases as cables get hotter

or is the Correction Factor just for Testing purposes ? OSG gives values at 10c and so when you test it, you adjust the Table value with the temperature at the time of testing to see if your results would be allowed

 

[Spark123]

I thought the correction factor would be so you can design circuits for a operating temperature, because resistance increases as cables get hotter

The maximum operating temperature of a cable is given by the manufacturer, if you need to limit the operating temperature of this cable then the cable manufacturer should be consulted. The resistance of a cable increases as it warms up, the maximum efli for a circuit is given by BS7671 at its operating temp.

or is the Correction Factor just for Testing purposes ? OSG gives values at 10c and so when you test it, you adjust the Table value with the temperature at the time of testing to see if your results would be allowed

The values in the OSG are for testing in an ambient temperature of 10ºC to 20ºC, the cable temperature not being above 20ºC in this instance.

 

[Jim2287]

Is the following correct ? ...


Tables in OSG are for 70c rated PVC cables
The table figures are the Maximum EFLI values at 10c Amb. Temp.

Scenario:

30 Amp BS3036, with 1.5mm CPC to disconnect in 0.4secs

EFLI from Table in OSG = 0.91Ohms

When I carry out the test the ambient temperature is 25c
The correction factor in OSG for 25c Amb. Temp = 1.06

So 0.91ohms x 1.06 = 0.96 Ohms

So on the day I tested at 25c, my reading for EFLI shouldnt exceed 0.96 ohms



Right or ?

 

[Spark123]

You've got me wondering how they arrive at these figures now, the OSG states that the figures are for a conductor at 10ºC yet they are 80% of the figures in BS7671 which to me relate to 20ºC. Maybe a good one for the boys over at the iee website?

 

[Spark123]

I have just had a look at the big brown book and a couple of threads over at the IET website, Appendix 3 in BS7671 gives the Maximum EFLI for a protective device as Zs = Uoc / Ia. Regulation 413-02-08 basically states that (itself) is considered to be satisfied if the condition Zs <= Uo / Ia is fulfilled, which is in effect another version of the first one, although as Uoc is nominally 240v and Uo is nominally 230v, the result is that the earth fault loop impedance will need to be lower for the latter equation to be sucessfull.
The OSG gives the maximum efli figures for Uo of 230v and the figures for max efli at 10ºC (and then they give a nice big ambient span of 10ºC to 20ºC), which are in the same region as 240v figures at 20ºC.

 

[Taylortwocities]

Rule of thumb, maybe??

 

[Jim2287]

Ok thanks for that, and the "Scenario" i gave, that is the sort of thing you would use the correction factor for ?

 

[Spark123]

It sounds OK to me