A question from a mate

[trev]

Hello all, long time no speak
I was talking to a mate of mine the other day about this place and he emailed me a question he set to a class of apprentices. I've not had a go at it (yet). Have a try-
3 single phase heating elements are fed from a 3 phase 4 wire 400 volt supply. Assume each element is 2KW at 230 V. The switch is in the neutral (once upon a time it was allowed)
Calculate the heat output
A with the switch open.
B with the switch closed
I have the answers but as I said above, I've not worked through it myself yet

 

[davesparks]

That is an excellent question!

 

[trev]

It is mate. It took a while for me to dust the cobwebs off
Plenty more where that came from.

 

[Darkwood]

Very good question indeed and I can see what reply your after which I'll leave to the Trainee's of course


TBH I would be interested to see this question in the main forum too after its run its course in here ... its a thinker for most levels :devilish:

 

[trev]

Is it within your mod powers to copy and paste it into open forum or shall I just reproduce it when the time comes mate?

 

[Darkwood]

Is it within your mod powers to copy and paste it into open forum or shall I just reproduce it when the time comes mate?

I know but was going to let it run as your little baby in here first ..didn't want to steel any thunder plus didn't want the Trainee's seeing the answer in the main forum which is the main reason to do it later

 

[Marvo]

Is this the circuit Trev?

View attachment 29990

 

[ElectroChem]

Nice one. I've got my mental answer, which depends entirely on whether they've used a switch before or after the star point.

 

[trev]

Think about what is written mate

 

[ElectroChem]

So, 6kW open and closed or did I miss the gotcha?

 

[Darkwood]

So, 6kW open and closed or did I miss the gotcha?

From your post 7 question I assume you have got the answer as it is the same question I would have asked (switch is N in and 3 N tails out)... so your answer to A and B is?

 

[ElectroChem]

My answer is that the switch will have no effect, they've driven the elements in star and the whole neutral wire could be removed with no change. 6kW.

 

[Marc Lajevardi]

could somebody explain to me how to work it out please?

 

[Darkwood]

could somebody explain to me how to work it out please?

With trev's agreement I'll ask Marvo to undelete his post which is a wiring layout to better explain what is happening, now that the answer is out there I don't think there is any issue to explaining indepth the reasoning behind the answer.

 

[Marc Lajevardi]

Ok thanks, if its too much to delete somebody could DM me?