8.5kw shower wires in 4mm, burnt terminals

[Pete999]

slightlyoff topic, but hre's something that has been bugging me for ages.

when applying the temp. factor ( 1.2 ) to your loop impedance as per osg., why apply it to the whole Zs value and not just to the R1+R2, when the supply cable from sub-station is never going to approach anything like 70 deg. ????

thoughts???

May be a good idea to add to the 18th consultancy debate Tel, but never given that much thought to it, make you think though, good point

 

[Bobster]

slightlyoff topic, but hre's something that has been bugging me for ages.

when applying the temp. factor ( 1.2 ) to your loop impedance as per osg., why apply it to the whole Zs value and not just to the R1+R2, when the supply cable from sub-station is never going to approach anything like 70 deg. ????

thoughts???

Because it's a rule of thumb from the OSG.

You can calculate actual operating temperatures if you wish, and derate using those values. However I don't believe this would be worthwhile in a domestic environment.

 

[Ian1981]

slightlyoff topic, but hre's something that has been bugging me for ages.

when applying the temp. factor ( 1.2 ) to your loop impedance as per osg., why apply it to the whole Zs value and not just to the R1+R2, when the supply cable from sub-station is never going to approach anything like 70 deg. ????

thoughts???

We do by doing the rule of thumb 80% values of those given in bs 7671.
This has already been taken into account
As there's a 20% difference in operating temperature between 20 degrees and 70 degrees
Example if we want to adjust our zs value of 1.37 say 10 degrees for a type B mcb then it's 1.37 x 0.96=1.31
If we want to adjust back to 70 degrees then it's 1.31/ 1.2= 1.09 ohms which is our rule of thumb value.

 

[Pete999]

slightlyoff topic, but hre's something that has been bugging me for ages.

when applying the temp. factor ( 1.2 ) to your loop impedance as per osg., why apply it to the whole Zs value and not just to the R1+R2, when the supply cable from sub-station is never going to approach anything like 70 deg. ????

thoughts???

May be a good idea to add to the 18th consultancy debate Tel, but never given that much thought to it, make you think though, good point

 

[Des 56]

Tel said when the supply cable from sub-station is never going to approach anything like 70 deg. ????

A few years ago a new build hotel I worked on had 3ph 185 mm at the supply
A Jcb driver managed to chop the suppliers incomer
What looked like no bigger than about a 16mm suppliers cable for the hotel was also supplying a lot of the houses in the local area,I would think that cable was one hot cookie when in full flow

 

[dmxtothemax]

The fact that only one wire was burnt suggests that the problem came from a loose/ faulty terminal or connection. Even thou the cable size is not adequate, it alone did not cause this, a bad connection caused this !

 

[Strima]

slightlyoff topic, but hre's something that has been bugging me for ages.

when applying the temp. factor ( 1.2 ) to your loop impedance as per osg., why apply it to the whole Zs value and not just to the R1+R2, when the supply cable from sub-station is never going to approach anything like 70 deg. ????

thoughts???

Which page of the OSG is the calc on?

At a guess they're trying to aire on the side of caution???. The actual design formula is Zs=Ze+1.2(R1+R2).

Problem is you get two different results. I've just plucked some numbers out of the air, Ze-0.35, R1+R2-1.12.

Using the design calc I get 1.274 ohms. Zs=Ze+1.2(R1+R2)

Using the OSG calc I get 1.344 ohms. Zs=1.2(Ze+(R1+R2))

So in some cases the incorrect CSA cable could be used, although this would only mean going up a size.