High-Leg Delta System Protection

Three-Phase Monitor Relays


Can I use a phase monitor for protection on a high-leg delta system?


Yes, Macromatic phase monitor relays provide the same reliable protection on high-leg delta systems.

A high-leg delta system (also commonly called red leg and wild leg among others) is a common configuration involving a transformer secondary having one winding center tapped with neutral and grounded. It is commonly used to provide two 120V AC and one 208V AC circuit with reference to neutral although other voltages are possible depending on the phase to phase voltage on the secondary windings.

In order to apply a phase monitor relay in this installation, it is important to realize that despite the 120/208V AC circuits available with reference to neutral, that the phase to phase voltage is actually 240V AC on each of the three phase pairs (Vab, Vbc, Vca = 240V AC). A phase monitor relay should be connected to phases A, B, & C and not to the neutral or ground connections. If using a phase monitor relay that requires selection of the Voltage, you will need to select 240V AC which is the phase to phase voltage of the system.

If your high-leg delta system operates with different circuit voltages, simply take the high-leg voltage and multiply by 1.15 to get the approximate phase to phase voltage used for selecting and configuring your phase monitor relay.

Note: High-leg delta power systems commonly suffer from unbalance due to uneven loading on the 120V AC circuits. Selecting a phase monitor with adjustable unbalance protection (PMP series or PMD series) will ensure the ability to compensate for existing unbalance. Properly adjusted, this will prevent nuisance tripping while still protecting your valuable equipment and machinery.

Please contact Macromatic for more details.