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A

atto2007

Hi guys,

struggling to work this question out and don't seem to be getting a correct answer no matter what cable conductor size i use.

Q. A 230V 14kW domestic electrical cooker, installed in a house is to be supplied by a 15m run of PVC insulated and sheathed cable, clipped to a surface. The cooker control unit has no integral 13A socket. The circuit is to be protected by a BS3036 fuse and the value of external impedance is 0.85 ohms. Calculate the minimum size cable that can be used for this circuit to comply with shock protection.

Ib = 14/230*1000 = 60.87A
In = 100A therefore max Zs = 0.53 ohms. ( i think)

Zs = Ze + [(R1+R2)*mf*L]/1000

Zs = 0.85 + [(1.83)*15] / 1000

Zs = 0.88 ohms ( too high)

using 50mm cable Zs = 0.85 + [0.387*15] / 1000

Zs = 0.86 ohms (still too high)

Feel im missing something obvious.
 
what he said^^^^. apply diversity and you get a design current of 27A
 
4mm cable. Let's see how you get there....

Assuming you use a 30A BS 3036 fuse the 80% Zs value is 0.87Ω.

Because of the high Ze, I've worked out you'd need 25mm² for live conductors and cpc to achieve the max Zs

4mm²/1.5mm² = 0.85 + (15 x (16.71 ÷ 1000)) = 1.1Ω (too high)

25mm²/25mm² = 0.85 + (15 x (1.454 ÷ 1000)) = 0.87Ω

The only way around it would be to RCD protect the circuit and rely on the RCD for earth fault protection as max Zs would then become 1667Ω. This way you could use 4mm² cable as you suggested.
 
my 27A came about because i use 1/3 remainder, 51/3 = 27 ( can't be arsed using one of them new-fangled abacus thingies with all them stupid buttons )
 
Assuming you use a 30A BS 3036 fuse the 80% Zs value is 0.87Ω.

Because of the high Ze, I've worked out you'd need 25mm² for live conductors and cpc to achieve the max Zs

4mm²/1.5mm² = 0.85 + (15 x (16.71 ÷ 1000)) = 1.1Ω (too high)

25mm²/25mm² = 0.85 + (15 x (1.454 ÷ 1000)) = 0.87Ω

The only way around it would be to RCD protect the circuit and rely on the RCD for earth fault protection as max Zs would then become 1667Ω. This way you could use 4mm² cable as you suggested.

Fair enough. I had it in my head for some reason (think it was the wine plus the statement of no socket outlet) that I was looking at a 5 sec disconnection time, hence I had allowed 2.0 ohms (OSG).
I now see the error of my ways.... :sad_smile:

Note to self...No posts after the first glass....
 
Last edited:
Q. A 230V 14kW domestic electrical cooker, installed in a house is to be supplied by a 15m run of PVC insulated and sheathed cable, clipped to a surface. The cooker control unit has no integral 13A socket. The circuit is to be protected by a BS3036 fuse and the value of external impedance is 0.85 ohms. Calculate the minimum size cable that can be used for this circuit to comply with shock protection.

Protection against electric shock we can assume in this case to be automatic disconnection of supply. Therefore we are concerned with Zs being appropriate to the protective device.
The Ze you are given is very high at 0.85 ohms, this is not considered suitable for a TN system, however the questions does not identify the earthing system. Assuming a TN system, disconnection time <=0.4s for a load less than 32A.

14000W at 230V gives a current of 60.87A, using diversity for a cooking appliance 10+((I-10)*0.3) gives a design current Ib of 25.26A.

Using Ib<In a 30A fuse is required.

Maximum Zs for a 30A BS3036 fuse is 1.09 ohms (at normal operating temperature).
Correcting for ambient temperature (0.8 rule of thumb or value in OSG) gives 0.9 ohms max Zs.

Your external resistance (Ze) is 0.85 ohms therefore the internal circuit (15m of PVC cable) must have an ambient temperature resistance of <= (0.9-0.85) <= 0.05 ohms.
Therefore resistance of the circuit cable = 0.05/15*1000 = 3.33 mhoms/metre
Using R1+R2 values from OSG the smallest cable with an R1+R2 value <=3.33 mohms / metre is 16mm line conductor and 10mm protective conductor. Since twin and earth comes with a 6mm CPC, you would need to use 16mm 3 core cable.

The result of this calculation tends to lead me to believe that there is an error in the question as some of the information is not required and some information is not given.
 

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