thermal constraint help?!

[willo1991]

first post on here, use it quite a lot and have found it extremely helpful so thanks to everyone who does post and reply i'm studying at the moment and have come across a question that im not too sure how to finish, the question is as follows;

A 230 volt 14kW cooker installed in a house is to be supplied by a 15m run of pvc insulated and sheathed cable, clipped to a surface. the circuit is to be protected by a BS3036 30A 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 thermal constraints.

so i start with;

Ief = 230/1.01 = 227.722
T= 0.1
K=143
Therefore S= sqrt227.722 squared * 0.1 / 143 = 0.503

where do i go from here? very confused?

any help or guidance would be greatly appreciated! again first post on here so go easy on me lol.

 

[telectrix]

where does the 1.01 come from? and k for T/E or bunched cables is 115, not 143.

 

[Richard Burns]

f
A 230 volt 14kW cooker installed in a house is to be supplied by a 15m run of pvc insulated and sheathed cable, clipped to a surface. the circuit is to be protected by a BS3036 30A 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 thermal constraints.

If you are considering only the thermal situation then this could be either that the size of the cpc must be able to handle the fault current or that the csa of the supply cable that can take the design current.

You have calculated the minimum size for the cpc but you have used the wrong values.
You do not have enough information to use easier values so you have to run with minimum standards.
S = root (I￾￾²t) / k
Fault current at origin is Uo/Ze = 230/0.85 =270.85 A, from BS3036 graph this is about 0.22s as Tel says k is 115
S = root (I￾￾²t) / k = 1.1mm￾².

For the minimum size of cable for the load:
Design current is 25.3 A with diversity so with 2.5 PVC surface clipped you can run 26A,
But this is less than the 30A protection so 4mm would be better at a CCC of 37A,
However with a 30A BS3036 fuse you need a CCC of 30/0.725 = 41A so a 6mm cable would be required.


[You are only designing in the question for thermal, however this does not consider the disconnection time that requires a Zs (at ambient temperature) of 0.9 ohms or 1.09 ohms at normal operating temperature, which 6mm cable (it does not say it is twin and earth so could be a 6mm CPC rather than 2.5!) would not meet and would require a 16mm cable to meet this Zs (which is ridiculous because of the high Ze)]

 

[nottz_spark]

first post on here, use it quite a lot and have found it extremely helpful so thanks to everyone who does post and reply i'm studying at the moment and have come across a question that im not too sure how to finish, the question is as follows;

A 230 volt 14kW cooker installed in a house is to be supplied by a 15m run of pvc insulated and sheathed cable, clipped to a surface. the circuit is to be protected by a BS3036 30A 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 thermal constraints.

so i start with;

Ief = 230/1.01 = 227.722
T= 0.1
K=143
Therefore S= sqrt227.722 squared * 0.1 / 143 = 0.503

where do i go from here? very confused?

any help or guidance would be greatly appreciated! again first post on here so go easy on me lol.

s = I2 multiplied by t divided by k....
s = minimum size cpc acceptable
I2 = operating current of protective device
t = time in which protection takes to reach operating current(I2)
k = multiplier value of conductor insulation

you will need your big green book for this one.

 

[ajelec10]

You will firstly have to work out the zs of the circuit to give your your minimum, fault current at the extremity of the circuit, you have been given the ze so you will need your on site guide to work out the resistance of r1+r2, it's a bit backwards as you will need to guess the size, it's states that its a insulated and sheathed cable so go for 6mm t+e at 15 meters. Then you can work out for fault current, then you can work out the actual disconnection time of the 3036. Put all figures into the adaibatic equation and it will indicate if the 2.5mm cpc in the cable is sufficient. 6mm t+e on a 30a is sufficient for a domestic cooker up to 15kw it states this in osg. Just in case people say otherwise. Hope this points you in the right direction. Ps also just woke up!! Good luck. AJ