Electrical course question help

[Jordanh1992]

An electric cooking appliance is to be installed in commercial premises. The circuit is wired using a 12m length of 6.0mm2 PVC sheathed cable clipped direct to a surface, and containing 2.5mm2 CPC. A 45A BS 1361 cartridge fuse protects the circuit and the value of Ze for the installation is given as 0.40 ohms. Assuming that there are no correction factors for grouping or ambient temperature, determine whether the circuit complies with BS 7671 for shock protection? The nominal voltage may be taken as 230 volts and the design current is 45A.

Ib: 45A
In: 45A
It: 45A
Iz: 47A. No correction factors.

VD= 7.3 x 45 x 12/1000 = 3.94
6.0mm2 is acceptable.

Max Zs= 1.04

0.40 + (10.49) x 1.02 x 12/1000 = 0.13
Zs= 0.40 + 0.13 = 0.53
Zs= 0.53
Zs is not above the maximum permitted value of 1.04 ohms and therefore is acceptable.

Have I worked this out correctly? Thanks again for all of you that help me out!

 

[Outspoken]

Looks good, although personally I would up it to a 10mm² if this was a real circuit as the design current and the capacity of the cable are way too close for comfort in my personal and humble opinion. I like to see a good gap of about 10% between the two at least.

 

[dave274]

Isn't the max zs of 1361 45a 0.52

 

[rmc86uk]

Is it not something as simple sayng BS1361 doesn't provide protection from electric shock?

 

[Outspoken]

The latest edition of BS7671 seems to be lacking information for BS 1361 fuses however these are basically the same BS88-3 and the figure for a 45A BS88-3 on a 5 second disconnect is 1.04Ω, 0.57Ω is the nearest quoted to your 0.52Ω and that is a for an 80A BS88-3.

I don't have a regs book with me at the moment, just a table with all the data sets in on my tablet, so if you can find a table for the BS1361 that I missed I will stand corrected.

 

[dave274]

Table b5 in the osg says 0.52 but obviously that's at ambient temp.

 

[Outspoken]

Dave, good call chap, double checked my tables and there I have Table B5 for BS1361 fuses. (really should have looked harder before) and you are correct, the maximum Zs for a 45A is indeed 0.52Ω, significantly different from a BS88-3. In my defense I have not dealt with a BS1361 for a good few years and not had a reason to look one up since college more than 25 years ago!

 

[Guest55]

shock protection verification is asking you to prove that the cpc is of sufficiant size to clear the fault current.
you do this with the adiabatic equation.
the question has given you all the values needed to complete the sums.
;-)

 

[GLENNSPARK]

The latest edition of BS7671 seems to be lacking information for BS 1361 fuses however these are basically the same BS88-3 and the figure for a 45A BS88-3 on a 5 second disconnect is 1.04Ω, 0.57Ω is the nearest quoted to your 0.52Ω and that is a for an 80A BS88-3.

I don't have a regs book with me at the moment, just a table with all the data sets in on my tablet, so if you can find a table for the BS1361 that I missed I will stand corrected.

thats because BS1361s are a redundant number....
you are correct for using BS88-3

 

[Outspoken]

Thanks chap, long time since I considered BS1361 and remembered that they are ostensibly BS88-3's...glad to know my memory is not as bad as I thought

 

[lauriehurman]

I agree with Biff.
Shock protection - so does the size of the CPC allow enough current to flow to trip the MCB in the time required?
Adiabatic equation.

Laurie