Phase to phase fault

If both circuits are using typical breakers, they have what could be considered solenoids built into each one. When the short occurs, there is a rapid inrush which triggers the internal mechanism, and opens the circuit. Sometimes only one breaker will trip. Sometimes aging changes the characteristics and its reaction varies.

The one time I saw a similar occurrence, both incoming supplies were equipped with differential protection. A differential relay sums all currents into and out of the breaker. In normal operation, the sum at any given instant should be zero. The differential relay will trip the breaker if the sum is other than zero. For example, a US Ground Fault Current Interrupter (GFCI) looks at current in the hot & neutral conductors, and will trip if the error exceeds 6 mA.

In my case (and possibly yours), the current to the improperly wired device left source 1, but returned to source 2. The utility would simply see unbalanced load, which is a common occurrence. The differential relay on each source would see a non-zero sum equal to the current drawn by the lamp and would trip.

In order to verify the actual cause, it would be helpful to know the type of protection for both supplies. Are they circuit breakers or fuses? If circuit breakers, what type of tripping is used? Is it overcurrent only, or does the breaker have a shunt trip with a programmable protection device? A one-line distribution diagram for both blocks, including the first utility connection point, would also be useful.

The problem seems to be the pole. Why will the lamp not burn?Any individual Breaker provided for the lamp pole? How has the Electrician Isolated the Lamp Pole? A single line diagram will give a better clarity to the problem.

The supply to the residential supply will trip if the relays are not set properly or if the downstream breaker closest to the lamp pole does not trip hence the question why will the lamp not burn?

I believe that there is insufficient information to make an educated decission regarding the cause of your problem. I also believe that if you are unable to supply sufficient information regarding the installation to allow a report to be made, then you will never understand the problem sufficiently to supply an accurate report.

There are many factors to consider and tests which should be done by some competent person:

What do you mean by a Phase to Phase fault? I can only assume that you mean the phase from one source of supply to the phase of the second source of supply, which may or may not be the same phase at the origin of the supplies.

The practice of using two different sources of supply for one circuit, or even on one set of controls, can be dangerous in itself, unles the load has some sort of identification advising any person working on that circuit, that it has two sources of supply connected, so that any person will not inadvertantly come into contact with a supply that they did not know they should have isolated.

Was the intial supply to the lamp completely and correctly isolated prior to the connection of a second supply?

A test should possibly be made of the Insulation resistance of both supplies as well as between both supplies, as there should be no electrical connection between the conductors of either supply or between the two supplies when isolated or connected to the load (Light). An Insulation Tester should be used for this test. The type and testing voltage being dependent upon the applied voltage of the system.

The type of protection used on the system?

If an RCD is used for protection, along with any other type of protection, then was the Neutral conductor also completely and correctly isolated. If not then you will continue to have problems with the RCDs tripping.

A neutral from another circuit can also lead to unsafe conditions in the supposedly isolated circuit. An incorrectly connected Neutral conductor is as dangerous as an Active conductor and can easily cause a fatal electrical shock.

There are other problems that can also occur with "mixing up" circuits that should never be allowed to occur, but a good knowledge of wiring methods and local wiring rules should prevent this from occurring.

The methods use in the installation will also alter the possibility of different faults occuring, as will the method or type of earthing system used.

I am writing from Australia where we use what is known as the M.E.N System of earthing. I know from many instances of finding wrongly connected circuits that if the earthing system is incorrectly connected, then it is possible this can also cause a fatal electric shock.

My reply may be of little help to you, but I am trying to point out that the problem may be very simple or quite complex, but would certainly need more complete and accurate information for any one to supply you with a satisfactory answer

Regards

Bill Pilgrim

Queensland, Australia