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Contactor and Switch Ratings

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Views since 12/05/2008

The following question/comment recently came up on the Citi-Car mailing list and I decided that it needed addressing in a more permanent fashion than just a reply.  Thus this web page.


>In my conversion, I got rid of the main contactor, and instead used a heavy duty master switch with a key. It works fine, and is one less component to maintain or circuit to depend upon. Also, I was told 1/2 speed reverse would work with 12v, but I don't think it does. Nor does it really matter.


I must urge the most extreme of caution in this area. I have yet to find a keyed heavy duty master switch that is actually capable of handling the kind of current we're dealing with.

There are two major questions.

1) can it handle the current. In most cases, the answer is no. There ARE switches capable of 500 amps continuous but they're relatively huge.

2) and much more important, can it INTERRUPT the high current that could be in the 1000+ amps range during a controller failure.

If the controller fails, and most failures involve shorts of the transistors and full power application, your first reaction will be to jam the brakes. They can't stop a Citi with the motor directly connected to the battery via shorted transistors in the controller but the brakes CAN push the current up into the 4 digit range.

With current that high and 48 or even worse, 72 volts behind it, it takes a HUGE and specially designed switch to break the current. Especially with the motor inductance there to kick the voltage up vastly higher as the current tries to change.

From my experimenting, I would expect such a switch to be blown apart or if the contacts don't move fast enough, to weld shut. If, in the process of blowing apart, the leads manage to make contact and weld together, then you're in the same situation as if the contacts weld - a vehicle the equivalent of a stuck throttle and no way to shut it off.

Think about how much "damage" only 100 amps from the arc welder (rarely with over 40 volts open circuit) does to the welding rod and metal being welded. That the "damage" is desired is irrelevant. A hundred amps at a few volts can melt a fairly large diameter rod and thick pieces of steel. Think about 10x that much current at work.

Here is a photo of some common DC switches and contactors.  Caution: The full size photo under the thumbnail is very large.  Dialup users beware.  I left it large because I want you to be able to see the differences in switches and contactors designed for EV use and some that are forced into that duty.  The little automotive contactor in the center is shown because I've seen so many of 'em on broken and not running Citi Cars.  I have to wonder if perhaps one of these welded its contacts and that resulted in the car being parked.

The switch on the left came from an AVS electric bus and controlled the main 12 volt supply.  Note that it is NOT rated for high voltage.  I can't find a spec on this particular switch but it is usually 32 to 36 volts or less.  Also note that it is not designed to interrupt even its full rated current.  It is a disconnect switch and not a breaker.

To get a scale of size other than from the little automotive solenoid, there is a bottle of iodine behind the Czonka holding it upright for the photograph.

This is the venerable Allbright SW200 fork lift/golf cart contactor that many people use in EVs.  I own a whole box of the darned things but I couldn't find 'em and so I borrowed the photo from EV Parts.  Let's look at its ratings:

  • Continuous Duty Rating:           250 amps
  • Maximum Intermittent rating:    360 amps
  • Maximum Interruption rating:     1500 amps
  • Maximum voltage:                    120 volts

This relay has shown itself  to be very rugged and capable of severe overload, especially if it is equipped with the optional blow-out magnets.  You can get an idea of the size of the contacts necessary to handle this kind of current in this picture.  The moveable contact is visible.  Lots of these are used on 144 volt EVs.

Before we go, a consideration of price is neessary.

  • Kilovac Czonka                    $216.00
  • Allbright SB200                    $133.00
  • Square D Breaker                 Approx $400
  • Manual interrupter switch      Approx $100
  • Car solenoid contactor          Approx $30.

You pretty much get what you pay for!

The Czonka and his big brother the Bubba

These reason these contactors have such outstanding specifications in such small packages (and such high prices) is that they use contacts placed inside a vacuum chamber.  A vacuum is the perfect insulator.  Until the voltage rises sufficiently (tens of thousands at the contact spacing involved such that field emission (electrons just jumping off the cathode) becomes a problem, a vacuum is the perfect insulator.  That's why this relatively tiny breaker can handle such current.

An industrial breaker such as the Square D one pictured is a perfect emergency interrupting device.  It has been tested to both NEMA and IEC standards to meet its rating which is much higher than what most of us will ever deal with.  It should be located remote from the passenger compartment and operated with a cable. 

Yeah, I know that , of necessity, we violate the generally accepted EV high voltage safety rules of not having high voltage in the passenger compartment in our Citi Cars, but no need pushing things even farther.  Besides, if this thing ever needs to interrupt full fault current during an "oh shit" moment, there is going to be a LOT of fire and flame and smoke for an instant.  At night it could flash-blind you for a few seconds which would not be good for driver safety.

Breaker Detail

A contactor or breaker must dissipate a huge amount of energy during the instant it is interrupting fault current.  Consider the Square D breaker.  If it interrupts 10,000 amps at 125 VDC, during the time the arc is being quenched, 10,000*125 = 1,200,000 watts or over a megawatt is dissipated inside the breaker.  Even though this lasts just a fraction of a second, many joules of energy are released.  This results in immense mechanical stress, almost identical, in fact, to an explosive detonating.  The result is a breaker that weighs almost 10 lbs to handle a measly 80 amps continuously.  This photo:

Shows one aspect of the design.  How the energy is dissipated.  The energy is in the form of a high pressure plasma of vaporized contact material and air.  It escapes the breaker through a labyrinth designed to dissipate energy through conduction and ablation and finally it exits the breaker through the arc chute.

This handles less than maximum faults.  The worst fault, the so-called "bolted short" (meaning the conductors involved in the short might as well be bolted together) dissipates more energy than this system of labyrinths and vents can handle.  For that event, the whole bottom of the breaker is designed to blow off.  You'll note that the brown phenolic fiberboard is held on with plastic push-pins.  These are designed to release under a designated amount of pressure and allow the panel to blow away.

The result of one of these events is a big black splotch of soot and vaporized metal underneath where the breaker was mounted.  I should point out at this juncture an important difference between NEMA and IEC ratings.  NEMA ratings require that the device continue to function after a full rated interruption, though damage may be sustained.  IEC ratings allow the device to be destroyed as long as it performed its safety duty.  That's why IEC rated contactors and breakers are so much smaller than NEMA. You get what you pay for.

In the instance of this breaker, the damage would be that the blow-out panel would be blown off.  The contacts might also have to be dressed or replaced.  The breaker could be restored to service.  An IEC rated breaker interrupting 10k amps at 120 volts would explode!  But in a safe manner.  Of course.  And I have some oceanfront land up here in Tellico for sale if you believe that.

I photographed that breaker in my hand to show just how big the darned thing is.  Unlike AC where there are 120 opportunities a second to extinguish the arc, DC breakers have to do it with brute force and magnetic blowouts.

You get what you pay for.