i would like some help understanding the ratings of a relay. It's a long post. Please bear.

I have a 3-phase voltage monitoring device (called device from now on), that monitors phase reversal, outage, assymetry etc... The device has a relay with the following ratings:

Relay Output : 1 C/O
Contact Rating : 5A (For ‘NO’) & 3A (For ‘NC’) @ 250 VAC / 28 VDC (Resistive)
Utilization Category:
AC - 15 Rated Voltage (Ue): 120/240 V, Rated Current (Ie): 3.0/1.5 A
DC - 13 Rated Voltage (Ue): 24/125/250 V, Rated Current (Ie): 2.0/0.22/0.1 A

This device is used to control a DOL motor starter. The starter has a contactor coil which comes wired by default between L1 & L3.
L1 straight to the coil (terminal A2),
and L3 goes through
T1-start button-T2,
T3-stop button & thermal overload-T4,
T4-coil (terminal A1).

enter image description here

Now, the easiest way to wire the device is to remove the wire between T2 & T3, and connect T2 to the common (15) on the relay and T3 to the NO contact (18). So when the supply is normal, the device acts just like the original wire between T2&T3. When there is a problem in the supply, the relay opens the NO contact, and the contactor coil is de-energized. The contactor coil is 57VA when closing 13VA when closed.

enter image description here

Now my problem is that the Line to Line voltage is 400V +/-5V. The monitoring side of the device is rated for this voltage (415V). But the relay that breaks the line for the coil is only rated for 240V AC from what i can understand. So, is it safe to wire it in the way i mentioned? What are the alternatives?

Please reply for any clarifications, since my terminology might be difficult to understand.

Thanks in advance.


1 Answer 1


Your problem is, when the coil circuit is without power the coil voltage drop is neglible. Because of that there is 415V across your relay terminals as soon the start button is pressed. It's not built for that and may draw an arc.

You could check if the coil can operate at 240V, and wire it between L3 and N instead of L3 and L1. If not, you have to use a different relay.

  • \$\begingroup\$ All models of the device come with a 240V relay, probably to keep it compact. \$\endgroup\$
    – Iyceman
    Oct 13, 2016 at 21:03
  • \$\begingroup\$ All models of the device come with a 240V relay, probably to keep it compact. The current coil is rated for 360V with operating limits of 280V-420V, but 230V coil is available separately. I will probably have to buy that. I was hoping to avoid the N, since it involves a bit of extra wiring. Thanks for your input. \$\endgroup\$
    – Iyceman
    Oct 13, 2016 at 21:05

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