Neutral Point of Heater Bank

If the heater is made up of groups of parallel banks of elements in each phase then it is essential that you connect the star point of the element banks to the supply neutral.  If you do not then if a single element in the parallel bank fails the load will become unbalanced and the 'star point' of the load will shift with respect to the star point of the supply.  This will cause a voltage rise on some of the elements and this in turn will lead to more failures.


This is the same situation as a lost neutral on a three phase distribution system that feeds single phase loads (unbalanced). 


You can calculate the voltage distribution using Millman's theorem.


I had to deal with a situation like this when I was an electrical officer in the merchant navy.  We took delivery of a couple of new lubricating oil heaters.  These were three phase banked elements connected in star - this is not a suitable design for most ships as the engine room services are generally connected to an IT system that does not have a distributed neutral.  A delta connection does not cause a problem.


The elements in these heaters started to fail, first  in one phase and then in a second.  The end result was complete failure in two phases and none in the third..  The manufacturer's rep was called in and after I demonstrated the fault using banks of 15 watt lamps and presented him with the calculations two new delta connected heaters were supplied on free issue - a very costly mistake on their part.


Regards


Geoff Blackwell

If the heater is made up of groups of parallel banks of elements in each phase then it is essential that you connect the star point of the element banks to the supply neutral.  If you do not then if a single element in the parallel bank fails the load will become unbalanced and the 'star point' of the load will shift with respect to the star point of the supply.  This will cause a voltage rise on some of the elements and this in turn will lead to more failures.


This is the same situation as a lost neutral on a three phase distribution system that feeds single phase loads (unbalanced). 


You can calculate the voltage distribution using Millman's theorem.


I had to deal with a situation like this when I was an electrical officer in the merchant navy.  We took delivery of a couple of new lubricating oil heaters.  These were three phase banked elements connected in star - this is not a suitable design for most ships as the engine room services are generally connected to an IT system that does not have a distributed neutral.  A delta connection does not cause a problem.


The elements in these heaters started to fail, first  in one phase and then in a second.  The end result was complete failure in two phases and none in the third..  The manufacturer's rep was called in and after I demonstrated the fault using banks of 15 watt lamps and presented him with the calculations two new delta connected heaters were supplied on free issue - a very costly mistake on their part.


Regards


Geoff Blackwell