Maximum current in a relay

Question

I have this datasheet for a Goodsky relay. It has 4 poles.

1. The datasheet mentions that the contacts are rated for 5 A. Is this the rating of a single pole? Does this mean the relay can switch 20 A if all the poles are connected to a single load?
2. In the banner it mentions 2P and 4P are "available with a switching current of 5 A. In the main text it mentions carrying current is 5 A. Does this mean that the carrying current and switching current are same for this relay?

Edit.

The load to be switched will be mostly a combination of household appliances for lighting, cooling (refrigerator, airconditioning,) etc. The relay would be ON all the time except in the case of a fault trip.

Answer 1

The rating is for a resistive load. You cannot parallel contacts to a single load to increase capacity. You should not exceed 5A, and de-rate for unpleasant (non-resistive) loads and/or if long life (in terms of number of operations) is required. For example it's not unusual to run a "5A" relay at 0.5A or 1A so that it lasts for a few years rather than a few months of continuous switching operation. For example, if the relay switches 1.5x per minute (say a controller application) 24/7 it will last only about 6 weeks at 5A even with a benign electric heater load.

It's rare to see relays rated for higher carrying current than switching current, even though some can handle higher current. This particular construction has all the load current flowing through a flexible wire, which may be the limitation. Basically switching current is limited by the contact materials and construction as well as the power used to move the contacts, and the carrying current is limited typically more by the flexure(s) is a relay. Once gold plated contacts have switched high power they are no longer useful for switching tiny currents.

The relay is from a major and long-standing Taiwan maker, and has a valid UL file number. It's not really a "power" relay though, more of an automation relay. If you want to switch amperes, I suggest a power relay. The datasheet should have curves showing the (typical) life for different load types and load currents.

Answer 2

You'll notice from the data sheet that the endurance is 10^7 no load, 10^5 rated resistive load. The endurance will be even lower for any load inductance (rise of voltage on breaking) or capacitance (current surge on connection).

The relay contacts are specified as gold alloy. This is a fragile material used for good contact resistance, not for endurance.

The name of the company is Goodsky. No, I've never heard of them either.

If you parallel poles, you will get higher carrying current. Do not assume that higher means N times higher, for N poles, as you can't ensure that the current divides equally.

If you parallel poles, you will not get higher breaking capacity. You cannot assume that all poles open at exactly the same time, one of them will always be the last to open.

Summary. If you parallel 4 poles, you may get 15 A carrying and 5 A breaking capacity, with only 10^5 operations expected into a pure resistive load, less into any interesting load like a motor or piece of electronic equipment.

Answer 3

1. The 5A rating is usually for a single pole. And no, it does not mean you can combine all poles to switch a 20A load, because one of the contacts is always the last to break or first to make contact so it would mean a single 5A contact would carry and need to make or break a 20A load.

2. If make/break current is 5A and carrying current is 5A, then they are the same. Sometimes some relays can carry more than make or break according to specifications, but not this.

Answer 4

 The load to be switched will be mostly a combination of household appliances

If possible, split your appliances in 4 groups with a total consumption of each group being close to 5A. Connect each group to a separate pole. Then you will be able to switch close to 20A and still stay within the specification. You can derate the poles (e.g. stay below 3A for instance) by using more poles or several relays, if you want extra lifetime.

Bridging the poles of a mechanical relay achieves very little in terms of maximum switching current: there always will be a pole which engages first and (possibly another one) which disengages last. During switching, that pole will carry the full current, which may be enough to weld it to a "permanently closed" state or oxidize it to an essentially "permanently open" state.