supply to 3 phase commando socket

[333rocky333]

Done some 3 phase water heaters recently, they were 3 elements in star, they had no neutral on the old ones.
However the makers said put a Neutral on the star point.
apparently its ok when all 3 loads are equal ,but when one load fails it causes problems with the other two.

A star or delta run motor or a star / delta motor would only need a neutral if it had a 240v control circuit
http://www.tlc-direct.co.uk/Book/4.3.12.htm

 

[333rocky333]

Nope, you don't need a neutral
The appliance will connect across 2 or 3 of the phases. There shouldn't be a neutral pin on the socket - it should be a 3p+e outlet.

You need a neutral for the test button to work though!

Untitled

• 333rocky333
• 27 Jul 2008
• 1

 

[Spark123]

Done some 3 phase water heaters recently, they were 3 elements in star, they had no neutral on the old ones.
However the makers said put a Neutral on the star point.
apparently its ok when all 3 loads are equal ,but when one load fails it causes problems with the other two.

A star or delta run motor or a star / delta motor would only need a neutral if it had a 240v control circuit
http://www.tlc-direct.co.uk/Book/4.3.12.htm[/QUOTE]

It would cause an issue as in the two loads will be then across the 415 circuit and therefore only have 207.5v across each heater.

A common practice which has been implemented on our site where 3 phase motors are used is to put a small transformer across 2 of the phases to supply the control circuitry.

 

[Spark123]

Nope, you don't need a neutral
The appliance will connect across 2 or 3 of the phases. There shouldn't be a neutral pin on the socket - it should be a 3p+e outlet.

Only correct for Delta.
If it's Star, then it deffo needs a neutral.

My reply was in response to Coljack, a 3 phase RCD doesn't need a neutral to function.
An appliance which does require a neutral to operate requires a 3p+n+e plug and outlet.

 

[333rocky333]

It would cause an issue as in the two loads will be then across the 415 circuit and therefore only have 207.5v across each heater.

The maker tried to explain it, but I got a bit confused.
Apparently quite normal for industrial heaters though, not that easy to add a neutral,as all kopexed in with 3 pole contactors etc.
The site was a bit worrying, they were 3 x 6 kw each unit, and controlled via a single phase stat and contactors, however each time the elements blew it welded in the contactors.
They fitted the new elements but failed to realise,luckily the isolater was damaged and they called us in for that.
otherwise the shower water would have been dangerously hot.

What would be the voltage across each element of 3 in star without the neutral, would that be 240 volt.

A common practice which has been implemented on our site where 3 phase motors are used is to put a small transformer across 2 of the phases to supply the control circuitry.

good idea, never did like 415v at the pushbuttons.
Got to do some star delta control stuff tomorrow.