JohdotaA phase failure relay is a monitoring relay that watches a three-phase supply. It detects a missing phase and a wrong phase sequence, and depending on the model, also a voltage that is too high or too low.
How it works
- When all the phases are present and in the right sequence, the output relay is energised: 11–14 is closed and 11–12 open.
- During a fault, the output relay drops out: 11–14 opens and 11–12 closes.
- If the phase sequence is already wrong when the supply is switched on, the output does not pull in at all.
Because the relay is energised in the normal state, a loss of the relay's own supply also shows up as a fault. That is why the NO contact 11–14 is wired in series with the motor control, and the NC contact 11–12 can switch on a fault light.
Other monitoring relays
A voltage monitoring relay works on the same principle, for example to detect a power cut in a standby power changeover circuit. In the normal state the relay is energised, and a fault or a power cut makes it drop out.
Frequently asked questions
What does a phase failure relay do?
It checks that all three phases are present and in the right sequence, and on some models that the voltage is not too high or low. On a fault its output drops out, and since 11–14 is in the motor control, the motor stops.
How does a phase monitoring relay work?
With the supply in order, the output relay is energised: 11–14 closed, 11–12 open. On a fault it drops out. Because it is energised in the normal state, a loss of its own supply also shows up as a fault.
What happens if the phase sequence is wrong?
The motor would run the wrong way, because swapping two phases reverses it. The relay detects the wrong sequence and its output does not pull in, so the motor cannot start.
How do you wire a phase failure relay?
Connect the supply phases L1, L2 and L3 to the relay. The NO contact 11–14 goes in series with the motor control, and the NC contact 11–12 can switch on a fault light.