Electrical switches

[Robert Johans]

Okay, thanks for that.

Back to my question: can I use a switch rated for 24vdc in a 12vdc environment "legally"?

[David Tilston]

If the switch is rated for 24 VDC. Is good for 12 VDC. The rating is a maximum. That being said. I would not exceed the 24 VDC current rating, unless it had a higher published 12 VDC rating.

[David Tilston]

Originally Posted by Byron Kinnaman

Switches are rated by peak voltages, a 120 Volt AC switch is designed so that it voltage is which it will arc is well above peak voltage, with in this case is 1.4 x 120.
As far as zero crossing, zero crossing does occur in AC voltages, but a mechanical switch isn't going to always switch as zero crossing, any more than it will always switch a peak voltage. MYTH.

Current rating is determined by the size of the contacts, AC or DC matters not. A 20 amp contact is a 20 amp contact no matter the voltage.

I know the myths are because you read it on the internet.

The two PDF files linked to are concerning going from a low voltage switch to a higher voltage circuit. Going that direction is a problem, going from a higher rated switch to a lower voltage is not a problem.

Byron, I think you may want to rethink you statements. A DC arc is much more difficult to extinguish. The zero crossing I refer to is not necessarily going to occur when the switch is thrown, but it will occur no more then 1/120th of a second after. This makes breaking the current much easier. In a DC application that zero crossing never happens.

I especally take issue with:

Originally Posted by Byron Kinnaman

Current rating is determined by the size of the contacts, AC or DC matters not. A 20 amp contact is a 20 amp contact no matter the voltage.

I am happy that you contradict this statement in your last paragraph. I am assuming that you are referring to the ampacity of the contacts. Which would be true, but it is current breaking ability that matters. If you have a highly inductive load, at a higher then rated voltage, the switch may not be able to break the arc, and a fire may result. Why would they be rated for current, and voltage, if this was not significant?

In high voltage switch gear, in a substation, they use huge quantities of oil or some other pretty exotic techniques are used to extinguish an arc.

I think to OP should consult a codes expert before they get a bad name for cutting corners against best practices.

[Byron Kinnaman]

Originally Posted by Robert Johans

Okay, thanks for that.

Back to my question: can I use a switch rated for 24vdc in a 12vdc environment "legally"?

I certainly think it's legal.

[Robert Johans]

David, I am confused with your final sentence of your second-to-last post. You write:
"I would not exceed the 24 VDC current rating, unless it had a higher published 12 VDC rating."
Can you elaborate?

Again, my goals in using low-volatage, DC-rated, 24VDC switches in my 12VDC environment are not only functionality and safety, but also legality.

Thanks again to all for your participation.

[David Tilston]

I was just cautioning against thinking that at 12 VDC, instead of 24 VDC, it may have a higher current rating.

[Byron Kinnaman]

Originally Posted by David Tilston

Byron, I think you may want to rethink you statements. A DC arc is much more difficult to extinguish. The zero crossing I refer to is not necessarily going to occur when the switch is thrown, but it will occur no more then 1/120th of a second after. This makes breaking the current much easier. In a DC application that zero crossing never happens.

I especally take issue with:


I am happy that you contradict this statement in your last paragraph. I am assuming that you are referring to the ampacity of the contacts. Which would be true, but it is current breaking ability that matters. If you have a highly inductive load, at a higher then rated voltage, the switch may not be able to break the arc, and a fire may result. Why would they be rated for current, and voltage, if this was not significant?

In high voltage switch gear, in a substation, they use huge quantities of oil or some other pretty exotic techniques are used to extinguish an arc.

I think to OP should consult a codes expert before they get a bad name for cutting corners against best practices.

Voltage and current ratings ==== Voltage rating is the highest voltage usable without the open switch arcing. Current rating is the highest current the contacts and conductors inside the switch can carry with heating up.

Arcing is going to happen, the ratings for cycles, that is on/off cycles, is a product of contact life or how long it will take to burn up the contacts.

Oil cooling it is used to keep the contacts cool so that more can be pushed through the contact than could in free air and has nothing to do with low voltage applications.

Switches are designed to open and close rapidly to reduce arcing and the time the contacts are in the arc zone.

Because the OP is concerned about low voltage operations with a resistive load I not going to go into all the switch design considerations of higher voltage switching. Nor will I go into different type contact material why different materials are used and when.

Also why you use reed switches or snap action switches, etc. For something that is seemingly simple there's a lot to it.

One more thing the arc is stopped by the spacing of the contacts. Since the maximum voltage in an AC circuit is 1.4 volts above the RMS (the voltage your meter reads) AC switches have to have a larger spacing between the open contacts.

You can believe what you like.

[David Tilston]

Originally Posted by Byron Kinnaman

Oil cooling it is used to keep the contacts cool so that more can be pushed through the contact than could in free air and has nothing to do with low voltage applications.

As a former sub station designer, I can assure you that you are incorrect on this one. Large blade style disconnects are used, with no oil, they do not require cooling, but they are not able to interrupt the current, (I have a great story on this). The oil in a bulk oil breaker is not for cooling. That would be a transformer.

But as you said, nothing to do with low voltage.

But, there is a current and voltage rating for a reason.

[stevebaz]

Originally Posted by Robert Johans

Okay, thanks for that.

Back to my question: can I use a switch rated for 24vdc in a 12vdc environment "legally"?

Legally speaking are you going to have a problem with 12 volt switch gear in a system charging anywhere from 12.7 volts to 15 volts? Looks to me like you need 24 volt switches to keep the Legal Beagles off your back should something happen.

[Byron Kinnaman]

Originally Posted by David Tilston

But, there is a current and voltage rating for a reason.

I explained that already.

I'm not sure where the "oil' you're referring to is located, around relay coils (contactor)? Surely not around the contacts.

Capacitors across the contacts are sometimes used to reduce arcing.

[Byron Kinnaman]

Changes in current ratings vs voltage ratings.

Automotive and heavy equipment switches made by Valeo specification on voltage/current "Electrical performance: From 20 mA to 16 A at 12V. From 20 mA to 8A at 24V."

The reason for the lower current at the higher voltage (24 volts) is these switches are rated for inductive loads. The reason these are listed this way is because they're used in such a manner that they almost always switch some sort of motor or relay. However this has nothing to do with resistive loads. Note the low current specification, the contacts are NOT gold therefore the 20 mA low end rating.

[Byron Kinnaman]

Originally Posted by Byron Kinnaman

I explained that already.

I'm not sure where the "oil' you're referring to is located, around relay coils (contactor)? Surely not around the contacts.

Capacitors across the contacts are sometimes used to reduce arcing.

If you can believe what's on the internet, this a quote for GEM manufacturing.

Contacts:
The contacts are immersed in oil bath which enables smooth and cool operation thus offering long life. The contacts made up of Electrolytic copper and brass are suited for vigorous duty. Wear and tear of the contacts are less, compared to air break starters, enhancing the life of the starters.

[Byron Kinnaman]

Originally Posted by Robert Johans

I am less than satisfied with the quality and aesthetics of typical 12VDC switches — both those for automotive use as well as RV.

Are there any good looking light switches (and cover plates) available anywhere that will function properly and safely with 12VDC power?

Here are a couple examples of the look I'm going for...

There's lots of options that you could use, look at electronic catalogs for switches and there's tons of them. I add some CCFL lights to my trailer and I'm 3/4" round rocker switches that I got a Radio Shack (shudder). I think you're using LED lighting, if so, almost any switch will work just fine therefore you can use styling as your main concern.

[David Tilston]

Originally Posted by Byron Kinnaman

If you can believe what's on the internet, this a quote for GEM manufacturing.

Contacts:
The contacts are immersed in oil bath which enables smooth and cool operation thus offering long life. The contacts made up of Electrolytic copper and brass are suited for vigorous duty. Wear and tear of the contacts are less, compared to air break starters, enhancing the life of the starters.

This is not really the place for a discussion on utility breakers, but bulk oil, min oil and SF6 breakers are about breaking the arc. And Yes, the contacts are immersed in oil. The SF6 breakers don't use oil.

The only point I am trying to make is that current carrying capacity is not the only design consideration. Interrupting capacity is also important. This is why the DC rating is different from the AC rating for a switch.

From the specs of a pressure switch that my company sells:
11 amps, 1/4 hp at 125, 250 VAC; 5 amps resistive, 3 amps inductive 28 VDC;
.5 amps resistive at 28 VDC

Note that 11 amps at 250 VAC becomes 0.5 amps resistive at 28 VDC

[Byron Kinnaman]

Originally Posted by david tilston

this is not really the place for a discussion on utility breakers, but bulk oil, min oil and sf6 breakers are about breaking the arc. And yes, the contacts are immersed in oil. The sf6 breakers don't use oil.

The only point i am trying to make is that current carrying capacity is not the only design consideration. Interrupting capacity is also important. This is why the dc rating is different from the ac rating for a switch.

From the specs of a pressure switch that my company sells:
11 amps, 1/4 hp at 125, 250 vac; 5 amps resistive, 3 amps inductive 28 vdc;
.5 amps resistive at 28 vdc

note that 5 amps resistive at 250 vac becomes 0.5 amps resistive at 28 vdc

just plan wrong...


Automotive and heavy equipment switches made by Valeo specification on voltage/current "Electrical performance: From 20 mA to 16 A at 12V. From 20 mA to 8A at 24V."

[Robert Johans]

Gentlemen, gentlemen!

Can we agree that, within our 12VDC travel trailer environment, a DC-rated, 24VDC switch is perfectly acceptable to control a bank of LED light fixtures drawing no more than 15 watts?

[David Tilston]

Originally Posted by Byron Kinnaman

just plan wrong...


Automotive and heavy equipment switches made by Valeo specification on voltage/current "Electrical performance: From 20 mA to 16 A at 12V. From 20 mA to 8A at 24V."

Byron,

What you have posted is just a comparison of two DC current ratings, lower at a higher voltage. Common sense. But the discussion is about AC ratings vs. DC ratings. If a switch is rated at 20 amps AC, it may have a much lower DC rating.

The specs that I posted are not "just plan wrong". They are determined by the manufacture through testing and research. Why do you say they are wrong?

[David Tilston]

Originally Posted by Robert Johans

Gentlemen, gentlemen!

Can we agree that, within our 12VDC travel trailer environment, a DC-rated, 24VDC switch is perfectly acceptable to control a bank of LED light fixtures drawing no more than 15 watts?

I would be comfortable saying that a 28VDC switch should be fine at 12VDC with the same current rating. At 15 watts, the current should not be much over 1 amp.

[steve dunham]

UL is a recognized testing and certifying lab. I installed a sign for a store in the Mall Of America. The sign came out of the south and did not have a UL sticker . I pointed this out to the store owner and the Mall of America . The store owner said install the sign over the objections of the Mall of America
The electrical inspector asked "Where is the UL sticker" and I explained the situation .Well we took the sign back out ,shipped it back to the manufacturer for reinspection and application of a UL sticker per UL standards
Electrical equipment is required to be certified by a recognized testing authority and the local enforcing authority (inspector) has the power to interpret the NEC . NO you can NOT just ship up a sticker and stick it on the equipment on site and be in UL compliance. This issue is a constant problem with pieces of equipment built overseas . I had a $6 000,000 German printing press sitting idle for 6 months waiting for UL certification Some parts of the country enforce the NEC others do not

[Joe MacDonald]

oooh oooh oooh..........sulfur-hexaflouride, I never thought I'd see that in a fiberglass trailer discussion!!!
I don't think you could fit that switch in a camper.

Here is a well thought out article that should clear up what is trying to be said: www.eaa.org/sportaviationmag/2005/0502_switches.pdf‎

I am an electrician, and I do wire in DC frequently. Read the rating, respect the rating, and use the right piece of kit. You might get away with something else, or you might not.

just my 2 cents