I have these contactors for my DC application. I never cared about which terminal to connect to HV, because I treated them like any mechanical switch (or 12V automotive relay).

But recently I noticed in this similar contactor's datasheet information about "reverse current". Then I checked datasheet of mines and surely, there is "Reverse load current direction" - I need to connect battery to A terminal and motor inverter to B.

Why is there a difference for contactor how to connect them? What "20x50A" could mean?

I need current to flow to both sides.

  • \$\begingroup\$ I found this in TE FAQs: Q: Why is load polarity important for a contactor? A: The breaking capability at high voltage is achieved by using magnets placed perpendicularly to the contact terminals. Both terminals are internally connected with a bridge. When the contact bridge moves away from the stationary contacts two arcs are generated. In the forward current direction, the magnets deflect the arcs to the outside resulting in a fast arc extinguishing. In reverse current direction the arcs could merge at the center causing a reduced breaking capability. \$\endgroup\$
    – Ri Di
    Dec 6, 2022 at 13:07

2 Answers 2


Why is there a difference for contactor how to connect them?

These are DC contactors and, with a current of several hundred amps the magnetic field generated could slightly increase or reduce the magnetic field of the actuation mechanism. Whether it increases the ability of the actuation mechanism to close the contactor or it decreases the ability will depend on the polarity of the current.

What "20x50A" could mean?

enter image description here

  • For forward current, the contactor is guaranteed to perform 5000 switch open operations at 100 amps and 800 volts
  • For reverse current, the contactor is guaranteed to perform 20 switch open operations at 50 amps and 800 volts
  • 1
    \$\begingroup\$ +1, but note that the three rows in each cell in the table are just three different ratings (cycle life vs current) for the same situation (they don't match up with the lines in the left column the way you highlighted it; the left column is just wrapped into three lines to fit). So it would perhaps be more instructive to compare the 50A ratings for both forward and reverse current (50 thousand cycles vs 20 cycles). Also note that the contactor is only rated to interrupt a 300V short in reverse. Basically, that's all about arc quenching, as explained in Spehro's answer. \$\endgroup\$
    – TooTea
    Dec 7, 2022 at 8:38

High current relays (or, more properly, contactors) typically have two stationary contacts bridged by a moving shorting bar with two contacts. That's because making a flexure or using a flexible wire capable of high current is fraught with difficulties.

High voltage DC contactors have an additional difficulty- the contacts tend to arc, especially if there is a bit of stray inductance in the circuit, and the arc can be sustained for some time and erode the contacts in a very few operations unless the contactor is made very large, and even then the arcing will continue until the contacts are far apart. (AC arcs tend to extinguish near the zero crossings, depending on how much stuff has been ionized and flung into the air, but in any case, easier than DC arcs).

A few ways have been used to mitigate this, such as pressurizing the switching mechanism with \$\text {SF}_6\$ (sulfur hexafluoride). As well as being troublesome and expensive, that gas is a powerful greenhouse gas (tens of thousands of times more than CO2) so it's only used where it has to be. Another way is to use permanent or electromagnets as "blow out magnets" to put a physical force on the ionized arc stream, extending the length and extinguishing it faster.

A good description can be found on the Durakool (of mercury contactor fame) website:

enter image description here

The purple cylinders represent the contacts. A DC current is flowing down on the left and up on the right. Two magnets are arranged to blow the arcs outward, away from the shorting bar with force F. Note that the 'hand' illustrated is the left-hand rule. enter image description here

Using the magnets allows high-voltage DC contactors and relays to be manufactured that are small and relatively inexpensive.

As the datasheet indicates, reversing the polarity can be worse than no magnets at all, and it's only rated for a a fraction of the current and much fewer operations (as low as 1 operation life for fault current).

  • \$\begingroup\$ With a density of 6.12 g/L at sea level, it's surprising that any SF6 can even make it into the atmosphere (very dense.) \$\endgroup\$
    – rdtsc
    Dec 6, 2022 at 14:06
  • \$\begingroup\$ @rdtsc EPA says \$\endgroup\$ Dec 6, 2022 at 14:13
  • 3
    \$\begingroup\$ @rdtsc The thing about gases is that once they mix, they're very difficult to separate. If you have 1 L of SF6, and it's exposed to moving air, then, much like in the book The Cat in the Hat Comes Back, that 1 L of SF6 is quickly going to turn into 10 L of 10% SF6, which will quickly become 100 L of 1% SF6, which will become 1,000 L of 0.1% SF6, and so on until it's completely dispersed throughout the earth's lower atmosphere. \$\endgroup\$ Dec 7, 2022 at 2:54
  • \$\begingroup\$ @Tanner-reinstateLGBTpeople just not quickly enough to avoid it being an asphyxiation hazard without good floor-level extraction. Anyway it's not the distribution of a gas in the atmosphere that matters for the absorption of radiation, but the total amount in a column. The distribution does affect where in the column the IR is absorbed, and that is of some relevance to climate change. \$\endgroup\$
    – Chris H
    Dec 7, 2022 at 16:11

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