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I want to install a relay to my fusebox which is controlling whole basement with machinery.

Relays that I have have got 2A limit each, and I have 32 relays. Can I just use all of them for the same line and assume the relay's limit is 64A ?

Is there any risk of doing this?

Nick Alexeev
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user2102266
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    Do all of your relays make contact within the same microsecond? If not, what do you think will happen to the first one that makes contact... – PlasmaHH Jun 18 '17 at 19:29
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    what if one fails? or two? or x... dormant failures. Paralleling is ALWAYS risky –  Jun 18 '17 at 19:39
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    worst case scenorio i can imagine is contacts in first relay getting fused. and it will pass current even if i try to open the circuits. Resulting bonus fireworks besides a simple pcb. – user2102266 Jun 18 '17 at 19:49
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    the downside is an extremely low MTBF. – dandavis Jun 18 '17 at 20:08
  • Domestic UK circuit breakers only go to 63A. How do you intend to protect this circuit, your basement and your dog? – Paul Uszak Jun 18 '17 at 20:41
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    Making some own insertions into the fusebox is a change to the accepted distribution system. The approval certificate of the original system changes to a piece of waste paper, if you are not a certified contractor yourself, who has power to do the work and accept the changes. Check the legal side even if you happen to know enough to put together a working system. Otherwise the bill can be astronomical. –  Jun 18 '17 at 22:03
  • You can certainly get relays with a higher capacity. – Hot Licks Jun 18 '17 at 22:57
  • @PlasmaHH, your comment REALLY needs to be an answer! – John R. Strohm Jun 19 '17 at 22:07
  • Good way to make an LER (Light Emitting Relay). – Samuel Jun 20 '17 at 22:24
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    Although this question is interesting on its own, it feels more like an XY problem... – Brad Werth Jun 21 '17 at 01:23
  • Related: https://electronics.stackexchange.com/questions/105126/is-it-good-practice-to-parallel-relay-contacts-for-increased-current-capacity – Stephen Collings Jun 21 '17 at 01:54
  • @PlasmaHH, not much will happen in one microsecond. – richard1941 Jun 22 '17 at 20:10

7 Answers7

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Unless you can guarantee that all the contacts will close and open at exactly the same instant of time the only safe current you can assume is 2A - that is, the capacity of the first contacts to close, or the last contacts to separate.

Majenko
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  • I can use 1 5v relay to power all other relays. Can i consider this as guarantee?. Can some of the relays may delay even if they get the current at the same moment ? – user2102266 Jun 18 '17 at 19:40
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    Since they are mechanical devices where an armature has to physically move, and the speed of that movement depends on factors outside of your immediate control, no you cannot assume that anything you do can make them all switch at the same instant of time. I'm talking femtoseconds or faster. – Majenko Jun 18 '17 at 19:42
  • Thank you . I guess i should use a servo and a switch :( – user2102266 Jun 18 '17 at 19:47
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    Or a high current *contactor* – Majenko Jun 18 '17 at 19:48
  • Those fancy things turn out to be more expensive then i first tought . i am trying to build a basic home automation system. After the proof of concept i would happy to buy those industrial big boy toys. – user2102266 Jun 18 '17 at 19:52
  • 64A is a huge current to be thinking about switching at home. Most houses in the UK are only fed with 80 or 100A master fuses... – Majenko Jun 18 '17 at 19:53
  • Most of the time 3kw is used but i have plasma cutter which is about 35kw and a small cnc desktop (220 volt). – user2102266 Jun 18 '17 at 20:00
  • That's some meaty stuff to run at home... – Majenko Jun 18 '17 at 20:02
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    I am struggling to find a 35kVA cutter that isn't 3-phase. The best I can find is 3.7kVA. Are you sure it's 35kVA and not 35A cutting current? – Majenko Jun 18 '17 at 20:11
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    This is totally a job for a contactor. It's simply not in relay territory. If you're controlling machinery you're dealing with inductive loading which is going to seriously downrate your relays as well. You simply can't do this with parallel relays. It won't work. – Ian Bland Jun 18 '17 at 20:13
  • @Majenko - Well, not femtoseconds. – WhatRoughBeast Jun 18 '17 at 22:29
  • @WhatRoughBeast maybe not, but faster than the time taken for 64A to melt a 2A contact... – Majenko Jun 18 '17 at 22:31
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    I don't understand. To me, "basic home automation system" means stuff like turning the lights on/off/dimmed, maybe a music system or some motorized blinds, perhaps a smart thermostat. How possibly does a basic home automation system involve plasma cutters and CNC machines? Any equipment you're going to be using with a 35kw plasma cutter is of a completely different character than what you'd use to control the bedroom lights. – Zach Lipton Jun 19 '17 at 05:22
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    @ZachLipton This sort of automation is what led to that bother with Skynet. – Paul Uszak Jun 19 '17 at 13:04
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    @ZachLipton You are not using enough imagination. If you have a decent CNC, you can easily modify your 35kVA plasma cutter into a PLASMA RAY GUN, the style of which has been seen on innumerable works of fiction, starting with the original Doom. Of course you are going to need a battery of relays, otherwise what would you wire your trigger to? Lex Luthor would be so proud. – Tobia Jun 19 '17 at 21:41
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    There's a second risk: The inner resistance of the relays. The manufacturer only guarantess that R10*R2 for two relays of the same type, and this would mean that the first relay takes almost no load. – yo' Jun 20 '17 at 17:49
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    @ZachLipton He is either doing something wrong, or something awesome. – Stephen Collings Jun 21 '17 at 01:57
  • @StephenCollings All intruders will be be scanned and prisoned in a custom made metal wearable cages. Cages will be fused so that prisoners digestive tracks will be aligned. I got this idea from a movie but it is too dirty to do by hand so i though i should automate this. – user2102266 Jun 23 '17 at 05:42
  • @ZachLipton its exactly what u think. I need to cut the power. As Majenko asked 64A is very high for a home. İf relays change state when these machines are on will box pop? apperantly the answer is "yes! eventually". – user2102266 Jun 23 '17 at 05:50
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Sixty-four little relays were turned on by a key;
One soon welded shut and then there were sixty-three.

Sixty-three little relays started glowing bright blue;
One of them turned black and then there were sixty-two.

Sixty-two little relays bore of amps a metric ton;
One said he'd had enough and then there were sixty-one.

...

(I can't be bothered to write 64 stanzas. I hope you get the idea. It ends with your house catching fire.)

Tobia
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Here's what will happen. When you activate your relays for the first time, the first one to close will have its contacts welded together by a massive overcurrent. Soon enough, other relays will close and hopefully distribute that current more or less evenly.

When you deactivate your relays, they will all open except the one with welded contacts. Since it will now be alone under full 64A load, it will soon blow up.

Now you have 31 little relays standin' in a line.

Dmitry Grigoryev
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You're not allowed to parallel.

You can't parallel through relays because you can't parallel generally.

In US NEC, it is disallowed except for currents so large that single wires are not readily available. I don't have a cite but I'm sure the EU model codes have the same rule, since it's a basic physics problem.

And even then, each paralleled conductor must be fused separately. (For instance I have an 800A 3-phase service with conductors paralleled, each on a 400A fuse. The conductors are 1000kcmil, the next size up is hard to procure.)

They must be the same material and length and terminated in the same manner, so they have the same electrical characteristics - that's to keep current from favoring one path over another. (which would result in a cascade of fuse blows).

This "favored route" problem will be even worse if you have contacts making (or breaking!!!) not quite simultaneously.

And remember, those small electronics relays aren't necessarily listed for use switching mains voltage. Their ratings as a component only mean they can be used as a component in an assembly/product/machine, which must then go back to the testing lab to be listed as an assembled machine.

Just use a big contactor. If your controller don't have the oomph to throw the big contactor, use a little relay to throw the contactor's coil current. This does an end-run around the snubbing problem; you only need to snub the little relay (though it will reduce arcing on the little relay's contacts if you snub the contactor's coil).

Or if one contactor is too expensive, you may be able to use several.

Multiple contactors are ok on multiple loads

I am inferring that you have more than one load. It's not paralleling to use one contactor per circuit or load. If each of your loads is <=30 amps, this is easy and cheap.

Suppose you have a 30A compressor, you can control that with a 30A contactor. As it happens, the HVAC industry has plenty of Code-rated 2-pole 30A contactors with 24VAC coils in the $12 range. 24V transformers are also in the $15 range. Here's a combo. Note how some of them are designed to mount in the cover or a knockout of a standard junction box.

If you have several loads on one breaker, you can just use another similar contactor on another circuit. If the contactor is 2- or 3-pole, you can switch one circuit per pole - that's not paralleling.

Harper - Reinstate Monica
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  • Really? You still have to snub the coil on a £300 100A contactor? I would have expected that to be built in for that price... – Paul Uszak Jun 19 '17 at 13:00
  • @PaulUszak You'd have to check the spec sheet. This is an electronics forum and to this constituency, that would make sense. The other users of that contactor might object to paying for snubbing they don't need. – Harper - Reinstate Monica Jun 19 '17 at 13:48
  • @PaulUszak: The optimal characteristics for a snubber will vary depending upon the circuitry feeding the coil. If the feed circuitry won't mind being hit with 2x the normal drive voltage, the coil may be able to switch much faster that if the peak voltage must be kept to 1.1x the drive voltage. – supercat Jun 20 '17 at 22:48
  • How cant i thick multiple answers :/ – user2102266 Jun 23 '17 at 05:52
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This is why they make large contactors (http://www.newark.com/c/automation-process-control/contactors-accessories/contactors) or for a good eg http://www.newark.com/eaton-moeller/dilm12-10-24vdc/contactor-24vdc-12a-din-rail-panel/dp/24M1369

Opto's traic/mosfet driven circuts or just throw on a solid state relay.

Now u can easily drive a large contactor like that any number of ways. I won't go into snubbers etc. But you had a good idea in theory to parallel the contacts but in practice you may/should see a cascade/premature failure, etc. These are the sorts of questions that lead people to eventually learn how to build & design professional systems :) Good stuff, But i'ma keep it short. Have fun! Feel free to ask more to clarify and i can drill down for ya.

Axel Stephan
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The answer to your main question is yes, there is a downside to using multiple relays to increase relay capacity, and it is - multiple relay failures, and a high probability of melted contacts and cables/wires.
As far as your second question, you can assume anything, but that does not mean that it is safe. The only safe thing you can assume, is that the relay bank will handle 2 amps (the design spec of each relay)!
If you want to be able to safely handle high currents, you have to get a single relay/contactor that can handle the required current. Sell your 32 relays and buy one contactor.

Guill
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I would opt for PLC's. That is where they shine. You can do that many and more with them and they are easily programmable. I would stay clear of the Fuse Box/Circuit Breakers however.

Tim Spriggs
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    Where did your find a PLC with a 64A switching capacity - 0.5A is typical? – Dale M Jun 21 '17 at 02:16
  • Put larger relays in between , on each branch circuit, that can handle it , Sherlock. Your thermostat controls a huge compresser/condenser and it can do it with a tiny relay. Same thing , just more branches of control, with a PLC. And if you need a contactor at the end of each one , use a contactor. The main thing here, is that you have control. Industrial control. The guy who asked the question should just contract it out to someone who has the necessary skills, for his own safety. – Tim Spriggs Jun 27 '17 at 19:12