# Why is a spring used in an electrical switch?

Note: I am not talking about a circuit breaker where a spring is used to counter the repulsive force generated by current passing through a small contact area.

This question is about switches designed for high currents (kA to MA range). The current pulse is DC, rising from zero to peak in milliseconds upon closing the circuit.

Generally, switches use a spring to open and close the circuit as fast as possible to avoid arcing. The spring force is also used to minimize bouncing of conductors when they strike each other. In short, a spring is used in a switch to create a toggle mechanism. This is my current understanding.

However, a contact force is also required to conduct a large current. This is from "Marshall's law" as applied to high amperage circuits (see here).

What is the exact role of a spring in a switch? What are the actual rules followed to design high-current switches? Any design codes or guidelines?

Suppose I want to conduct 100 kA of current through a conductor-conductor junction, what minimum contact force should be applied at the junction so that the current passes properly? (Please don't cite formula for blow-off force here.)

• I think you need to specify the approximate waveform for this, principles can change quite dramatically for AC signals, DC and pulses.
– Juan
Commented Sep 12, 2022 at 4:29
• You've told us why force is used in a switch. Are you asking why the force is supplied via a spring? For instance, instead of using pnuematics, or a rubber band, or gravity? Commented Sep 12, 2022 at 4:37
• No, I am asking the exact role of springs, Is it to close and open circuits rapidly or to provide a contact force to conduct electricity or both...? Commented Sep 12, 2022 at 4:44

## 2 Answers

I will give you a small introduction to the way of thinking for high current, I have small experience from high voltage applications.

essentially a switch has to be able to completely stop the current and not be destroyed in the process or damage other components, so avoiding spikes and other phenomena as best you can, both have to be repeatable.

in the range you specify, there is actually no way to avoid an arc, you basically design around it and try your best to actually interrupt the arc itself

I know you mentioned DC, but I will give you an example first with AC power lines so you understand how the difficulty ramps up.

so you disconnect a power line, when you physically separate the contacts an arc will form, this circuit has enough energy to keep the arc alive, but you have one big things working to your advantage, AC signals pass through 0 so if you can keep the contacts separated until that happens, you basically got a shot.. on top the arc itself warms the air and pulls the arc up making it longer and requiring more energy to mantain, this also works in your favor. Probably have seen a video like this

When you connect the opposite happens, when you get the contacts close enough an arc will form and when you get the two in contact, the arc will disappear.

Now you mentioned a DC case, here you have no guaranteed passes through 0 so when closing or opening the switch you need to do something about the arc, in some cases you have a gas that has good isolation capabilities in a controlled chamber... or a vacuum, example for opening, it can also be a liquid.

The big reason you want to suppress the arc as much as possible is that you wish the contacts to survive as many operations as possible, most of these high current switches have a set amount of times they can switch.

So to sum up this very brief introduction:

In very high currents for mechanical switches an arc will happen, you do your best to suppress it.

There are many designs and methods, if this book link works for you then great, you can familiarize yourself with more principles as you advance.

As for your contact force question, you can basically model the contact as a resistor, the better contact you achieve the more you can lower the resistance of the load circuit and the more current you can get.

I am not even gonna pretend I gave you all the answers to cover this topic, it is just a very simple introduction, it is a very difficult and long topic.

• Thank you for such a simple explanation, I am also studying a book by Paul G Slade and also another book by Ragnar Holm. In addition I have read a few journal articles as well that discuss design of switches. So far, it is clear that springs are used to rapidly open and close circuits while also making sure contacts don't shatter when they strike each other. so the only question is: do i need a minimum contact force to conduct current? What is the general consensus over contact force vs current when it comes to designing switches? Commented Sep 12, 2022 at 5:09
• Suppose i want to conduct 100kA of current across a conductor-conductor junction, What minimum contact force should be applied at the junction so that the current passes properly? Commented Sep 12, 2022 at 5:20
• I cannot answer that off the top of my head, it will change depending on the materials, say if both are aluminum and if both are copper it will change, what I can tell you is that you can actually apply pressure to a controlled set of both metals and measure the resistance vs the pressure and eventually it will stabilize, as in the resistance will not increase much even if you apply much pressure. I reckon you ideally would want that much force as a minimum. keep in mind that the junction has to be able to resist the heat from the current so your closing switch isn't a spot welder
– Juan
Commented Sep 12, 2022 at 8:53

The spring does two things:

1. reduces the risk of bouncing when closing - as that can cause arcs to damage the contacts,

2. the cam and spring combine to open the contacts quickly again due to the issue of arcing.

So design of switches is very complicated and you can research further but this gives you an idea.