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?
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.
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.)
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.
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.
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.
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.
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.
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.
