Arc Suppression

You should never put suppression components in parallel with relay contacts. If they fail short circuit then you have a load unintentially energised. There is also the matter of leakage currents through the device. An AC motor load should not require contact suppression. A DC motor load is probably best fitted with MOV across its terminals. If you are switching a DC inductive load then you need DC rated relays. Otherwise you can have high voltage tracking occurring in the relay which is another hazard to be avoided.

Hello Adrianjs,

I have seen arc suppression components connected across relay contacts. Usually they are RC network. Generic values from several sources suggested value fror resistor equals half the or equal to the applied voltage in ohms and the value for the capacitor around 0.1 micro fared with a voltage rating two to three times the applied voltage to ensure that the capacitor will not fail. The charge and discharge rate will be limited by the resistor and the peak current (Instantaniuos) will equal to the applied voltage X 1.414 devided by the value or the resistance.

The current to the load when the contacts are open will be a maximum of applied voltage devided by the impedence of the RC net work which will be less than 11 mA if a 150 ohm resistor and 0.1 micro fared capacitor for RC network and the applied voltage is 277 VAC. If by any chance the capactor fails closed, the low wattage (1/2 W) resistor will fail open acting as a fuse. Alternately, if the resistor failed close, the capacitive reactance of the capacitor (Approximately 26K) will limit the current to slightly over 10 mA @ 277 V, 60Hz,even if a short circuited load is considered.

My understanding is that motor load is inductive and therefore when switched OFF heavy arcing will take place across the contacts.

Regards,

Job Thykkoottathil

Hi,

If you are using an AC motor then the zero crossing of the AC supply kills any arc. DC is another matter and if you are using that then you need relays deisgned for switching DC loads. The suppression components should be across the load, not the relay contacts. You can end up giving someone a nasty electric shock as a minimum with the components that you describe above.

As an example of AC motor switching using a relay, I have a sawbench with a 500 watt induction motor on it that uses a small contactor to switch it on and off. In use for over 30 years I have never had to change the contactor.

Best wishes

Adrian

Best wishes

Adrian

use the very accuracy arc protection relay. i suggest you to use the sweden brand very good to preotect the equipemnt