Interested in using a switch that is rated at 20 amps for 30VDC (6A 250VAC and 15A 125VAC). How would this translate to 12-14VDC?

 

Got a data sheet for the switch? You need to know the voltage and current ratings of the switch contacts.

Interested in using a switch that is rated at 20 amps for 30VDC (

It doesn't translate as the 20 Amps 30 VDC are maximum ratings so 12 to 14 VDC is below maximum.

Ron

 

Let's see if I understand the issues involved in switches .....

First there is the situation when the switch is closed ... the contacts and connectors inside the switch will have a resistance , and this causes heat inside the switch dependent on the square of the current ( the DC voltage is immaterial) So 21A will create nearly double the heat 15A will ... this could cause runaway ( temp of switch rises , resistance rises causing MORE heat)

Then there is the issue of voltage , on closing the switch a spark can jump and weld the contacts together , but since your voltage is low , this is not relevant...

Then I think there is a difference between AC and DC ???? Not sure about that!

 

Then I think there is a difference between AC and DC ???? Not sure about that!

 

Let's see if I understand the issues involved in switches .....

First there is the situation when the switch is closed ... the contacts and connectors inside the switch will have a resistance , and this causes heat inside the switch dependent on the square of the current ( the DC voltage is immaterial) So 21A will create nearly double the heat 15A will ... this could cause runaway ( temp of switch rises , resistance rises causing MORE heat)

Then there is the issue of voltage , on closing the switch a spark can jump and weld the contacts together , but since your voltage is low , this is not relevant...

Then I think there is a difference between AC and DC ???? Not sure about that!

In addition to the great video above nsaspook was kind enough to provide switches have become en entire world unto themselves. Contacts are normally plated and the material be it gold, silver or a lower cost alloy plays a roll in not just how much current the contacts can handle but their longevity as well. Really good higher end switches have data sheets with everything you could want to know as well as things we never knew existed or mattered. On a much larger scale and pretty much off topic here is a good clip of a high voltage switch gear opening hot, 115 Kilo volts three phase.

http://www.capturedlightning.com/frames/mpg/345kV_SWITCH.MPG

Ron

 

^^^ That's a very curious video !!!

So , on attempting to open the switch with DC running , the current "has momentum" and doesn't want to stop ... jumps the gap , ionises air which makes the air conductive enabling the switch to be open an inch with current still running ...

I guess the way DC switches work is SPEED??? A quick disconnection prevents the arc being established ??? But why??

 

^^^ That's a very curious video !!!

So , on attempting to open the switch with DC running , the current "has momentum" and doesn't want to stop ... jumps the gap , ionises air which makes the air conductive enabling the switch to be open an inch with current still running ...

I guess the way DC switches work is SPEED??? A quick disconnection prevents the arc being established ??? But why??

I don't know that I would word the effects that way. Arcing is current flow through a gas and that would be true of AC as well as DC current. However, with an AC arc the current is constantly changing direction so the AC current is going through zero and that directional current change, while it will not stop an arc on the first cycle the arc becomes more difficult to maintain. DC current, however, is constant less the direction change of AC so maintaining an arc becomes an easier occurrence. The video nsaspook linked really shows that well since we can see the difference with the same resistive load (heater element).

Ron

 

^^^ That's a very curious video !!!

So , on attempting to open the switch with DC running , the current "has momentum" and doesn't want to stop ... jumps the gap , ionises air which makes the air conductive enabling the switch to be open an inch with current still running ...

I guess the way DC switches work is SPEED??? A quick disconnection prevents the arc being established ??? But why??

The electric field moves the charge carriers. As the switch opens a small air gap is created. When the distance of the initial gap is small in relation to the breakdown voltage of air across X gap we have ionization (electrons are stripped from the air molecules) of air. These electrons and ions absorb electrical energy from the electric field causing them to heat and accelerate to the metal switch plates. The energetic charges hitting the conductor heat the metal causing the generation of more free electrons and ions for the arc channel in a hot emission cathode for a current flow avalanche ion event as more electric field energy is absorbed. It takes time for this process to complete to a full blown thermal plasma arc condition. With AC or a fast opening DC switch the times/space/field strength integral of arc power is reduced from what a slow moving DC switch connect would be so there is less of a change to develop this cathode ion generation avalanche condition for a powerful and continuous arc in those cases. If we increase the voltage and/or really slow down the switching there will usually be a point of sufficient power in the initial arc to generate ion cathodes (bright spots) on the switch contacts for a 'continuous' high density current arc.

We can control this thermal plasma arc condition with magnetic fields, gas pressure regulation, etc... and use it to transport ionized particles to coat/treat other conductors.