AC Short-circuit Protection

Question

I have a bunch of electro-mechanical relays on a board intended to switch 220VAC for CFLs. The maximum load on the relays will be 100W with inrush currents of upto 2A (lasting for less than a second). The relays do survive short-circuit currents, briefly, as long as a 10A circuit-breaker kicks in.

The relays is Omrom G5LE-1A DC5 - a 240VAC 1200VA relay. I am also considering the G5LE-1A-E-DC5 which is the 16A version of the above relay with 4000VA switching power - link to datasheet.

My question is, is it safe to rely on this for a commercial product? We won't be installing this anywhere that does not have a proper breaker system anyway. I don't want to include a fuse for two reasons:

1. The product is intended to be mounted on a wall. The board will not be easily accessible.

2. The board has live voltages and if possible, I would prefer that the user does not have to replace the fuse very often, unless there is catastrophic failure like the circuit-breaker failing as well. I'm not counting a short-circuit as a catastrophic failure because some (poorly made) CFLs end up failing as a short-circuit and this could be a annual occurrence or more frequent - the user would probably get fed up of the product.

I think some sort of backup protection should be there but resettable fuses seem to be too slow, at least for mains voltages. What are my options here?

Answer 1

My favourite relay for this type of application is the NAIS / Aromat JS1 family. This is a form C relay with the NO contacts good for 10A and the NC contacts good for 8A. We have used several hundred thousand of these relays over the past 20 years or so driving many different loads, including water circulating pumps for hot water heating systems. No relay failures that weren't caused by installer wiring errors.

You don't need a fuse if everything connected to the AC Mains supply in your circuit is designed to handle the full current rating of the branch circuit that the unit is tied to. That means wide traces on the PCB.

Most of our boards don't have fuses or any other form of over-current protection on the AC Mains side of things. The low-voltage stuff in our HVAC boards uses a polyswitch self-resetting device to protect the power transformer should someone make a wiring error that would cause the power transformer to self-destruct. We use certified Class 2 transformers that have permanent (one-time) over-current and over-temperature cutouts. The polyswitch saves the transformer from dying because of a silly wiring mistake.

We use a larger relay when needed: American Zettler AZ2150 (form C) or AZ2150A (form A). We use those for controlling, among other things, 3/4 HP induction motors used for fans and blowers.

Answer 2

From the Omron datasheet, I would be inclined to say 2A inrush is nothing for the relay. The maximum capacity chart says 5A switching current and 10A AC continuous current.

You might consider a resettable fuse if you are concerned.