AC relays - get relay that can handle double the voltage?

Hello,

I'm a bit bewildered about relays. I'm aiming to relay 220v 5a max, so for this purpose I've thought of getting a relay such as the Omron G5Q, for example. Yet I've read some recommendations that one should be aiming for a relay which can handle at least double of the voltage one plans to switch on or off. I'm wondering what your take is on that notion...?

These folks - http://digistump.com/wiki/digispark/tutorials/relay, if only to name an example - don't find it relevant to state the above voltage-handling warning. And that appears to be a reasonably proffesional relay board they've put together.

So I'm a bit confused about what I should get & would much appreciate your take on it.

anything that comes from your wall can vary easily, I have plugged in and seen 110v one day, 130 the next. Some saftey margins should be considered when designing anything that involves electricity, but double seems a bit heavy for voltage, my concern would be current

you plan on 5 amps at X volts, I would look for 10, maybe 7 or 8 or whatever is above 5 and available and that is thinking of inrush current, and spikes in voltage

Normally people use relays rated for 120vac service in 120vac service and so with 240vac relays in 240vac service. Where one might often double the rating used Vs the actual service is in current ratings where it never pays to run relay contacts at 100% of their ratings if one wants long life and reliability. A lot depends on how often the relay is switched on and off over it's life time and the ambient conditions it has to operate in.

Lefty

Get a relay with contacts rated for the voltage you want to switch. That's what a specification is for.

I don't know what the supply tolerances are in US, with Osgeld saying he's seen 110v one day and 130v the next, but here in the UK the stautory limits are 230V +10%/-6% giving a legal variation of 216.2V to 253V.

It used to be 240V +/-6% so similar limits at 120V would allow 112.8V to 132V, so it's quite possible depending on time of day and load factors.

You'd still just get a relay rated at 120VAC though.

Mains-rated components are rated for far above the nominal voltage, since mains is known to carry transient spikes from fluorecent lights, fridge motors, lightning. Normally a "240VAC" rating means mains 240V rated - the possible exception being computer power supplies which are notoriously brittle in the face of real-world mains electricity and need anti-surge-filters to protect them (most these days have some protection built-in).

Oh BTW 240VAC is an r.m.s. value, the amplitude of "240V mains" is actually 340V (ie the live varies from +340V to -340V and back again over each cycle).

(In america "220V" mains is I believe two-phase 110V, so the peak voltages are actually +/-155V, but the difference between the phases does get up to 310V)

I'd say that unless you know you have particularly bad mains (light bulbs regularly blow, computers fail without good filtering) its not worth worrying about.

tack: here in the UK the stautory limits are 230V +10%/-6% giving a legal variation of 216.2V to 253V.

It used to be 240V +/-6% so similar limits at 120V would allow 112.8V to 132V, so it's quite possible depending on time of day and load factors.

No, I believe the situation hasn't changed - UK mains is still 240V, european is (mainly?) 230V, the official "230V +10%/-6%" is a bureaucratic convenience for people making products that work both in UK and Europe...

(In america "220V" mains is I believe two-phase 110V, so the peak voltages are actually +/-155V, but the difference between the phases does get up to 310V)

While it is often miscalled 2 phase, most U.S. households are more properly classified as 240vac 'split phase', where the center tap of the utilities 240vac secondary winding wire to the home service panel becomes the 'neutral wire' common, (and also grounded at the service entry) and either line leg is 120vac to neutral or 240vac from line to line. That allows easy wiring of either 120vac or 230vac circuits in the house, just by what connections the branch circuit breaker makes inside the service panel.

Lefty

MarkT:

tack: here in the UK the stautory limits are 230V +10%/-6% giving a legal variation of 216.2V to 253V.

It used to be 240V +/-6% so similar limits at 120V would allow 112.8V to 132V, so it's quite possible depending on time of day and load factors.

No, I believe the situation hasn't changed - UK mains is still 240V, european is (mainly?) 230V, the official "230V +10%/-6%" is a bureaucratic convenience for people making products that work both in UK and Europe...

The statutory limits are as I stated. Officially we are now 230V but, you're right, in reality nothing has changed. The 'new' figures of 230V +10%/-6% give almost the same voltage limit values as the 'old' figures of 240V +/-6%

When installing substations I still aim for transformers at Tap 2 or 3 which gives me around 247V-250V on no load at LV source. That will be dependent on expected LV load plus 11kV & 33kV load, distances to Primary/Grid substations and 33kV/132kV tapping schemes.

Got it - will go for specs + a tad bit more. Thanks a bunch for chipping in.

Are you switching an inductive load or a resistive load? Inductive loads require a higher rating because of inrush current and possible arcing on release. Resistive loads have little or low inrush and thus don't need to be as heavy for a given load. Often on relays there are 2 ratings, one for motors (inductive) and a resistive rating. If you are switching a 5A motor you want a relay that is rated for 5 A inductive, or 10 A resistive. If you are switching the motor on and off rapidly you might want to go higher as the average load might then be much higher.

As far as voltage rating. Get the nominal voltage rating for the supply. 120VAC is used for switching line voltage that is nominally 120VAC. Can run from 110VAC to 136VAC and is still considerred 120VAC for specification purposes. If you are switching nominal 240 volts then get a relay with that rating.