Dc on Ac switch

[ches2443]

Carlings has a FAQs web site. On that site they state that 125-240vac amperage rated toggle switch can be used to control dc current at a max 30vdc.
Has anyone tried this. And if so can it be a rule of thumb with other manufacturers.

 

[markstg]

Q: What DC current and voltage will an AC rated switch handle?
A: The DC current ?rule of thumb? holds that the highest amperage rating on the switch should perform satisfactorily up to 30 volts DC. For example, if you have an F Series toggle switch which is rated at 10A 250VAC, 15A 125-250VAC, the DC rating is 15A up to 30VDC.


The above is what the FAQ actually says. Hmmmm "Rule of Thumb"..."should"
IMHO Doesn't sound like something you can bank on.

 

[ultramegabob]

I am not positive so correct me if my thinking is wrong here... as far as I know "amps" are "amps" whether it is A/C or D/C, the issue with the voltage rating is the arc gap in between the contacts.

 

[Besoeker]

I am not positive so correct me if my thinking is wrong here... as far as I know "amps" are "amps" whether it is A/C or D/C, the issue with the voltage rating is the arc gap in between the contacts.

Yes, I think so.
We sometimes use ac contactors in a dc circuit. They are usually more compact and delivery period is shorter than bar and shaft DC contactors. Typically we use four-pole contactors with two poles on each side of the DC. And we always get a spec from the manufacturer on DC breaking capacity so that we can determine safe use for the application.

 

[LarryFine]

I am not positive so correct me if my thinking is wrong here... as far as I know "amps" are "amps" whether it is A/C or D/C, the issue with the voltage rating is the arc gap in between the contacts.

The electrical difference is that DC is much harder on contacts. On AC, the voltage drops through zero 120 times per second, which helps to extinguish arcing as the contacts open.

DC causes much greater contact pitting. Placing two sets of contacts in series greatly increases contact life. It reduces the duration of the arc by acting as if the contacts are separating faster.

 

[ultramegabob]

The electrical difference is that DC is much harder on contacts. On AC, the voltage drops through zero 120 times per second, which helps to extinguish arcing as the contacts open.

DC causes much greater contact pitting. Placing two sets of contacts in series greatly increases contact life. It reduces the duration of the arc by acting as if the contacts are separating faster.

I knew I was kind of on track about the differnce between A/C and D/C and the arcing, thanks for expanding on it...

 

[Jraef]

The current rating for a set of contacts is tied directly to its ability to SAFELY dissipate the heat created by current flow AND opening or closing within the duty cycle specified. So as others mentioned, when opening a circuit DC draws an arc for longer and that heats the contact material up a lot more than AC does. So because every situation is different and every manufacturer solves design problems in different ways, a rule-of-thumb is pretty much meaningless. The only safe way to do it is to get the DC capacity information from the manufacturer.

 

[broadgage]

A commonly applied "rule of thumb" is that switch contacts, relay contacts, contactor contacts and small circuit breaker contacts, That are marked "AC only" can in fact be used on DC but only at about 10% of the rated voltage.

For example it would be generaly accepted that a light switch marked "240 volts AC only" can be used on DC but only up to about 24 volts.
Likewise an industrial switch marked "AC only, 720 volts" could in practice be used on DC but only up to about 72 volts.

It must however be stressed that this is only a "rule of thumb" and that such use may well violate the NEC or other codes and regulations.