Relay Contact Rating 1

[charz]

The relay name plate mentions the following,
Making Capacity: 10A
Breaking Capacity: 2A
There is no continuous current rating mentioned.
I have my load operating continuously at 4A. Would be there any problem breaking this current of 4A? If so, the continuous current carrying capacity of the load should be less than or equal to breaking capacity in general?

 

[davidbeach]

Relay doing what? What's the circuit? AC or DC?

 

[charz]

It is the DC coil relay. the load is also DC

 

[charz]

Some manufacturer specify for example,
Continuous rating: 10A
Make Current: 60A
Break current: 6A
My question is when the continuous current to the load is 10A, since the breaking capacity is only 6A, will this affect the contact?

 

[waross]

If the continuous current is greater than the breaking current rating, then some other device should be used to interrupt the current.
A possible example may be a multi pole relay used to change the connections on small motor for reversing. The contacts are switched "Dry" and start-stop and overload functions are controlled by a single pole contactor. There are probably other instances.
Is interrupting rating important. It may be.
I was called out on a service call to a small DC powered hoist that would not apply the brake. The contacts had failed to interrupt the current, ever. A small arc was established across the open contacts of the double break relay that did not extinguish. The arc eventually melted the silver alloy contacts. The molten silver eventually flowed together and the arcs were shunted by a metallic path. There were two tiny hourglass shapes made of silver alloy where the contact openings used to be. I kept the contacts and the Ectachrome slides but I haven't seen either the contacts or the slides for many years now.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Often you need a lower breaker current then operating current in that the trip solenoid should drop out when the breaker opens- of course in my case I always spec high current interrupting contacts on all relays just in case.

 

[cranky108]

Or you could accept a lower contact life, or use a solid-state device to extend the breaking rating.

The second option is what I would do, but somehow the tech's get confused when I do that. Maybe they don't read the included instructions.

 

[jraef]

The "make 60, break 6, 10A continuous" rating that you see is for AC circuits. For that reason, the maximum inductive load, often expressed as a HP or kW load, will be lower than the continuous rating because it is based on the 6A portion. The reason is, the inductive load will sustain the arc when the contacts open, increasing the heat significantly. If the load is resistive, the contacts can handle the breaking better because the voltage will pass through zero twice per cycle, which helps extinguish the arc faster that with the inductive load.

Why this relates to you is that if you have a relay designed specifically for DC loads, that rating is based on the same concept of arc continuance, because with DC there is no issue of the voltage passing through zero to help. So in effect, your DC continuous amp rating IS the break rating. Your relay is rated for 2A. The Make rating can only be used if something ELSE in the circuit will be interrupting the current flow under load. For example if you have this relay feeding some sort of power converter such as a chopper drive for a 2A load, applying power may involve capacitor charging current in the instant you turn it on, so the 10A Make rating would apply, but if you ever shut down, the chopper drive actually turns the load off first, THEN the contacts open with vitually no load, so the 2A Break rating is fine. In the absence of some other controlling device, you can only use that relay at 2A.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

In the case I mentioned, the current was much much less than the AC interrupting rating, but as jraef points out on the inductive DC circuit there was no zero crossing to help extinguish the arc.

Bill
--------------------
"Why not the best?"
Jimmy Carter