Don’t Get (Single) Phased Out


Most larger buildings and factories are supplied by three phase services at voltages that range from 208V to 575V. Without getting in to too much detail, a three-phase system is used because it’s generally more economical than an equivalent single-phase system. The three phase system also uses less conductor material to transmit electric power than an equivalent single phase system at the same voltage and is therefore is less expensive to install.

Much of the electrical equipment and many of the motors in these buildings are also fed with three-phase power.  Under normal conditions this is a very reliable, simple and robust means of feeding these components.

As many building managers and operators are aware, a disruption of the power being supplied by the local utility can wreak havoc with the building equipment. There can be a variety of reasons that the power is disrupted and often these disruption results in a temporary loss of one phase in the three-phase system.

During this phase loss the current draw on the remaining two phases is increased and in most cases the overcurrent protective devices feeding the equipment will sense this change in current and disconnect the motor from the service before any damage can be done.

Ok then so what’s the problem?

Well, in reality we often find that many of these motors starters are not able to disconnect the motors quickly enough and when the power is returned to a normal three phase state the building operator or manager can find that there are many damaged motors that will need repair or replacement.

If this has never happened in your building it all sounds like a lot of technical theory.  If you have experienced this you know it is very disruptive and expensive to find yourself without critical pumps, fans and other motors.

One of the simplest solutions is to install a single-phase protective device on critical equipment.

This detector is sometimes compared to a smart fuse or to an intelligent protection relay: it continuously monitors the voltage level and phase-angle on each phase of your electrical supply. If a phase failure (single phasing) occurs, the 3-phase monitor will immediately open the control circuit of the protected equipment, thus preventing damage. When the voltage returns to normal, the three-phase monitor resets itself and closes the control circuit, allowing the protected equipment to be started normally and safely.

This is a fairly economical and simple means of solving this problem.  If you would like further information about this or any other power quality issues please give me a call at 416-490-8093.

Yours partner in electrical building safety and management,


Mark Marmer