2
\$\begingroup\$

It's around five decades since I had to do anything with mains control, so I just need to clarify something.

I need to build some standalone units to allow me to remotely disconnect the 240 V supply from equipment in the event of a fault. I plan to use readily available 12 V DC coil board-mounted non-latching relays with DPCO contacts rated at 10 amps at 240 V AC. The control circuits are no problem.

The equipment (PCs and an audio desk) will be connected via the N/C contacts and I do not expect to have to disconnect them frequently (preferably not at all!). AC current drawn will only be a couple of amps, but I can't say what any inrush current might be. Each load will have its own relay switch unit.

Although the C/O contacts are over-rated for their normal use, do I need to build in any contact protection for when the relay breaks the supply? If so, what do I need?

I know about the DC coil protection, and that is built onto the boards I intend to use.

No problem with the construction, everything will be in screened and grounded metal boxes, it's just this old brain hasn't kept up with modern ways!

Any advice gratefully received.


Further details:

Each 240 V AC controlled item is currently fused at 5 amps and does not pop fuses so I guess inrush currents won't be a problem.

The relays will be breaking the 240 V supply, not making it, so I'm interested to know about anything that will minimise contact damage on break due to arcing etc.

The units are for emergency use only, usually to remotely unlock a PC that Microsoft has locked up by a "critical" update that doesn't complete and is therefore off the network and unavailable.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ The one thing you can't say anything about could be a showstopper (inrush current). \$\endgroup\$
    – Andy aka
    Feb 4, 2022 at 16:05
  • 1
    \$\begingroup\$ PC's and some audio equipment have large inrush current requirements - anything with a switch-mode power supply., \$\endgroup\$
    – rdtsc
    Feb 4, 2022 at 16:09
  • 1
    \$\begingroup\$ Chrisathome - Hi, You confused people by adding an edit to the answer :-) On Stack Exchange, when you have generally-applicable new information, you edit your question to add it. Do not edit an answer to add new information. In simple terms, think of it this way - the question "belongs to you", the answers don't. What you can and should do, is when you make an edit to the question which is relevant to any of the answers, is to add a short comment below each relevant answer e.g. "I've updated the question to add more info about X - does your point Y still apply?" I hope that's clear. \$\endgroup\$
    – SamGibson
    Feb 4, 2022 at 22:08
  • \$\begingroup\$ [continued] While it is sometimes appropriate to respond in a comment to an answer (note, that's still not an edit to that answer), that should only be done if your reply would not be generally applicable to other readers. If your response is relevant to other readers, then you should edit the question, add the new info there, then (if appropriate) add a short comment below any answers which are affected by that update in the question. Adding that comment below an answer, will trigger a notification to that answer's author. Then they can choose whether to update their answer, or not. \$\endgroup\$
    – SamGibson
    Feb 4, 2022 at 22:12

1 Answer 1

2
\$\begingroup\$

If the steady state current is only "a couple of amps" then there should be no problem with breaking it with contacts rated at 10A @ 240V AC. The inrush current might be a problem and you might have to consider including some protection in the form of Negative Temperature Coefficient (NTC) thermistors. There is a useful application note here.

However most contact damage (arcing) tends to occur when they are breaking circuits not closing them.

Notes on "Further details"
Whether or not the 5 A fuse blows depends, in part, on it's type. A fast blow fuse could be taken out by a heavy inrush current, whilst a slow blow fuse could withstand it.

If the relay is already closed when the power is applied the contacts should easily cope.

As already discussed breaking "a couple of amps" should be no problem for contacts rated at 10A, particularly if it is only a rare occurrence.

Contact arcing is only a real problem if you are switching DC and the load is inductive (unless we are talking about serious AC voltages and currents where contactors and specialised contact configurations are required) none of which appears to be the case here. Nevertheless if you are still worried you could try googling "Protecting relay contacts against arcing" and review snubber networks, but these are mainly aimed at the aforementioned DC/inductive scenario and may introduce their own problems. For example any using a diode will almost certainly not work for AC.

\$\endgroup\$
1
  • \$\begingroup\$ Peter Jennings - Hi, In lieu of a comment from the OP, just FYI they have added some more info in the question, if you weren't already aware. (The new info was initially edited into your answer by mistake and has since been removed.) \$\endgroup\$
    – SamGibson
    Feb 4, 2022 at 22:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.