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"DC Rule of Thumb"

2510 Views 3 Replies 3 Participants Last post by  boekel
Duh! I should have just Googled it!

"DC Rule of Thumb
For those switches that list an AC voltage rating only, the "DC Rule of Thumb" can be applied for determining the switch's maximum DC current rating. This "rule" states the highest amperage on the switch should perform satisfactorily up to 30 volts DC. For example, a switch which is rated at 10A 250VAC; 15A 125VAC; 3/4HP 125-250VAC, will be likely to perform satisfactorily at 15 amps up to 30 volts DC (VDC)."

Check 'em out, (Carlin Technologies) they have lots of good/interesting info.

Here is lots more interesting info:
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I believe that's sizing for contact heating only.

The switch will not last as many switching cycles switching the same DC as AC.

My wet finger in the air (since we're using rule of thumb) says substantially fewer.
Still looking into this (diggin' deeper), I sent an email to Carling Technologies.

I've read your, "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."
My question is: will the "rule" extrapolate any farther? for(up to 72VDC)ELV circuits?
I ask because there are many scooters & e-bikes that operate @ 48VDC & 60VDC.
The circuits, to turn the speed controllers on or off, is usually pack
voltage with a less than 50mA draw.
So, does a switch with an actual DC rating (ex. 250VDC @ 10A) need to be used
or should most 250VAC 10A/125VAC 15A rated switches be able to handle 48VDC @
up to.5A or 60VDC @ up to .5A?"
Thanks, Kevin

Their response:

"Hi Kevin,
the reason the "rule of thumb" stays below 30V is that possibility of damaging arcing is much higher as the voltage potential increases.
48V and 72V systems should use switching mechanisms that provide a quick-make quick break.
Testing we have done on typical vehicle rockers (V series) show amperage at higher voltages will decrease
20A 12V, 15A 24V, 4A 36V testing at 48V showed the switch design not suited for 48V applications.
I would suggest a relay be controlled by the switch, or a DC design be used."

I asked,

"Hello Paul,
Thanks for your reply.
I guess I'm trying to find out if there is a minimum threshold of amperage @ 48VDC (or 60VDC) to be able to create an arc?
When I tested the amp draw of one of these 60VDC On/Off circuits the meter showed a 14.5mA draw.
(In my original question, I rounded to 50mA for an added safety margin)
Then, I did a simple "scratch" test of the bared wires, I could not see a visible arc. (even with the lights off)
My reasoning is: If there is no arc, then there should not be any damage to the contacts of a switch.
Does this make sense?
Thanks again for your help, Kevin"

Their response:

"Hi Kevin,
That theory is sound - but it would have to be tested.
We have no data on it, other than the tests we have done in the past
showed arcing, so I assume the draw was higher."

Then, I asked:

"Hello Paul,
Do you guys still do testing (R&D) to develop new products?
Testing to determine the minimum threshold for arcing, at different amperages, may be of interest & helpful to your company in developing new products.
…& to add to/extend your DC Rule of Thumb
As I said, there are many ELV (up to 75VDC) systems now available.
They all need low amp DCV rated control switches like on/off switches, brake lever switches etc.
...but, there are not a lot of switches available that are rated in this range. (up to 75VDC & lets say up to ~3A)

Just a suggestion,
...but, if you guys do any testing like this, please let me know/share the results.
Thanks again, Kevin"
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