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As known, contactor is used to switch higher capacity than relay. But there some relays can switch high current too, such as, some power relays can switch current beyond 100A, and there are contactors to switch only 160A. So, if a relay has a same switching current with a contactor, which one to choose?

And can relays be used in parallel to achieve high switching current to replace a contactor?

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    \$\begingroup\$ "Contactor" is a subcategory of "relay". A relay large enough to switch 100A certainly earns the "contactor" name. There is no "either-or", just like you can't ask "car or sedan?". \$\endgroup\$ – Agent_L Jul 3 '17 at 10:23
  • \$\begingroup\$ But usually, a contactor has different structure to a relay. And I mean this kind of contactor. \$\endgroup\$ – diverger Jul 3 '17 at 11:50
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    \$\begingroup\$ electronics.stackexchange.com/questions/311693/… may give insight when paralleling relays... \$\endgroup\$ – Paul Uszak Jul 3 '17 at 12:14
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    \$\begingroup\$ I know this is old, but on that last part of your question, I'd offer a definite yes. I've been using the 4 available 10A contacts in an OMRON relay (LY4-12VDC) to create a single higher current relay, to switch a 120VAC motor. The OMRON engineers I spoke to said it was definitely a good idea, and the fact that my very active application (often switches the motor 10X each day) has been running trouble free for 3 years now should offer some solace. \$\endgroup\$ – Randy Sep 14 '18 at 23:54
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Wikipedia's Contactor article explains it pretty well.

Unlike general-purpose relays, contactors are designed to be directly connected to high-current load devices. Relays tend to be of lower capacity and are usually designed for both normally closed and normally open applications. Devices switching more than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors. Apart from optional auxiliary low current contacts, contactors are almost exclusively fitted with normally open ("form A") contacts. Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents. [Emphasis mine.]

Further down the same article ...

Differences between a relay and a contactor:

  • Contactors generally are spring loaded to prevent contact welding.
  • Arc-suppression
  • relays usually have NC contacts; contactors usually do not (when de-energerzied, there is no connection).

enter image description here

Magnetic suppression and arc dividers are typically utilized when switching multi-horsepower motors. Magnetic suppression is accomplished by forcing the arc to follow the longer field lines of a fixed magnet placed in close proximity to the contacts. The longer path is specifically designed to force an arc length that can’t be sustained by the availableinductive energies. Figure 3 shows a schematic representation of magnetic arc suppression. Source: Automation Direct, Electrical Arcs - Part 1 of 2 part series.

The article linked above is well worth a read.

Your questions:

So, if a relay has a same switching current with a contactor, which one to choose?

Look carefully at the application and contact rating - particularly for motor or inductive loads. If you are satisfied that either will suffice you can choose based on some other criteria such as cost.

And can relays be used in parallel to achieve high switching current to replace a contactor?

Generally not. While doing this does reduce the long term heating of the individual contacts due to steady current running through them it is a problem during switching due to timing differences. Even wiring contacts of the same relay in parallel is risky as they never are perfectly aligned and the first one to make and last one to break carry the full switching action.

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  • \$\begingroup\$ Contactors often have auxiliary contacts or interlocks for low current control voltage. On a relay, there's no need to distinguish primary from auxiliary contacts. For instance in a series-parallel transition, P1 may close as S1 starts to open, but P2’s coil may be interlocked so it cannot start to close until S is fully open and landed. \$\endgroup\$ – Harper Jul 3 '17 at 21:09
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Load voltages maybe? Both contactors and relays can be used for switching high current loads, but contactors can be used for high-voltage applications as well. Besides, contactors do have some protections (like overload) on-board.

About paralleling relays: Can you guarantee to turn on all the relays at the same time? Can you imagine what will happen if one turns on before all the others and remains on even for a short time? I'm not telling that it will burn or fail, but depends on the relay's contact performance. Just to consider...

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  • \$\begingroup\$ Yes, contactor usually have higher 'switching capacity' than relay. Higher voltage or higher current both lead to higher capacity. \$\endgroup\$ – diverger Jul 3 '17 at 11:53
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EM contactors are quite close to relays, so I advise to check the specs rather than deciding upfront that you want a contactor. More powerful kinds which work with compressed air can't be practically substituted by relays, but it doesn't seem like you want those.

Connecting relays in parallel have been recently discussed, the short answer being no.

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Contactors tend to be a more 'professional' product, better specified, easier to use in a system, more expensive. The system designer will tend to use contactors, the module designer will tend to use relays, for reasonable meanings of module and system.

Contactors are more likely to be specified to be properly off when off, maintaining adequate contact separation to permit working on downstream equipment, which relays will not be.

Relays can be put in parallel to improve their carrying current, if you dress the lead lengths properly.

Relays should never be expected to improve on their switching current if used in parallel, because of timing. One set of contacts will be making first, or breaking last, and it's this set that will see all the wear, and fail very early. If it fails short, you'll notice, but if it fails open, you won't, as another relay will take over on point. This is the so-called 'zip fastener' failure mode, as one by one, all the damage is concentrated in each relay in turn.

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  • \$\begingroup\$ But usually we need another 'relay' to control contators, as some contactors can only be driven by AC. So drive it may not as easy as driving a relay. \$\endgroup\$ – diverger Jul 3 '17 at 11:47
  • \$\begingroup\$ Also, in some spectacular circumstances, firecracker mode. \$\endgroup\$ – WhatRoughBeast Jul 3 '17 at 14:26
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Generally I expect a contactor to pull a conductive "bar" or bridge down onto two contacts, whereas a relay has a single contact for each circuit and leaf which is hinged at the other end.

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  • \$\begingroup\$ It would be nice if your answer was correct, but I have seen many a contactor with only a single pair of contacts per phase instead of two as you mention and there is the occasional relay with two. \$\endgroup\$ – hildred Jul 3 '17 at 18:24

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