Arcing in electrical relay contacts due to different types of loads

[srinaath]

TL;DR

Arcing occur in a relay's contact due to closing and opening action. Does the arcing depends on the type of load? For example will there be any arc in a resistive load?

Arcing in a contact occurs due to open and close action of the switch. I am curious if the arc intensity depends on the type of load. For example the arcing (arc intensity) due to induction load will be greater than arcing due to resistive load?

 

[.Scott]

You can definitely get a lot more arcing with an inductive load.
As the circuit is interrupted, the voltage across the contacts rises.
In the worse case, you have a current source with enough reserve voltage to continue the circuit even when the relay is fully open.
The same problem exists with circuit breakers and regular mechanical switches.

 

[srinaath]

You can definitely get a lot more arcing with an inductive load.
As the circuit is interrupted, the voltage across the contacts rises.
In the worse case, you have a current source with enough reserve voltage to continue the circuit even when the relay is fully open.
The same problem exists with circuit breakers and regular mechanical switches.

Thanks @.Scott
quick clarification though - do we even see arcing in resistive load?

 

[Dullard]

You do get some arcing with resistive loads - it's due to the fact that contacts can be 'close enough' to ionize air very near actual contact. Most contacts also 'bounce' a bit. for AC switching, polarity changes tend to 'extinguish' the arcs. For HV DC contactors, arc extinguishing is a major design consideration.

 

[.Scott]

You may be interested in this article:
Intrinsically Safe (explosive environment)

 

[Windadct]

Also - while the load may be resistive - the system will have parasitic (unintended) inductance.

 

[Baluncore]

quick clarification though - do we even see arcing in resistive load?

Yes. Because the wires delivering the current to the relay contacts, and the wires connecting the resistive load to the relay contacts have some inductance.

 

[.Scott]

Even if the load is not inductive at all, the mechanical process of separating the conductors can draw an arc.
In fact, when I look at a physical contact that is breaking a resistive load on the oscilloscope - and spread the timeline out to nanoseconds, I can see many makes and breaks - as the contacts slide across each other and finally separate.

What the induction does for you is provide a voltage surge just as the break occurs. But you may already have sufficient voltage to provide an arc. Aside from wear and tear on the contacts and the potential to trigger an explosive gas, arcing will also result in a very wide band of EM emisions.

 

[anorlunda]

When we close a contact, the volts/meter gradient in the air gap must approach infinity as the meters in the gap approaches zero. That is until we get some electrical breakdown (an arc) or the height of molecular irregularities of the surface become significant.

As @Dullard said, closing contacts also bounce. If we ionize a few molecules of air on the initial contact, that makes it easier to create an arc on the second and subsequent bounces.

So as others have said, even with DC, the arcing on switch contact surfaces is nonzero.

When we open a current carrying switch, then $L\frac{dI}{dt}$ becomes an additional or dominant factor, but even without L, the volts/meter across the gap begins at very large values.

But when low voltage relay contacts have a short lifetime, look for inductance as the cause.

 

[Nik_2213]

Classic 'Bad Load' is 'traditional' small electric motor. It has just enough inductance for a lot of make/break arcing, which erode the brushes and commutator. You may hang a 'catcher' capacitance or back-to-back zener diodes across the motor terminals, which some-what limits the voltage excursions. This also mitigates EMP and line noise...

 

[dlgoff]

This Spark suppression .pdf file may be useful to others.

To prevent the contacts from being eroded a RC network is placed across the contacts. When the contacts open the applied voltage is placed across the capacitor and not the contacts. The capacitor is to charge up at a rate faster than the contacts open thus preventing an arc from forming across the contacts.

 

[Frodo]

The point about an inductor is that it tries to keep the current flowing through it constant ... which means the current tries to flow through the now open switch contacts.

The voltage across the open contacts rises until the medium breaks down and allows the current to flow.

Years ago I used an oscilloscope to measure the voltage spike generated when a tiny 12V reed relay was switched off. It generated an extremely short duration 1.5kV spike on top of the 12V rail. Adding a tiny catching diode limited it to 0.7V ... but held the relay closed for much longer.

Check the voltage and current ratings for switches or relay contacts. You will see that the DC current rating is usually far smaller than the AC current rating. As has been said, AC extinguishes the arcs at the zero voltage crossing thus reducing damage to the contacts.