Can two relays be connected in series or parallel?

image Partial voltage when relays are connected in series

When used with a DC load, connecting relays in series divides the voltage in two and it may be possible to interrupt a larger voltage than with a single relay. For example, if two relays with a rated switching voltage of 200 VDC / 20 A are connected in series, it may be possible to interrupt 400 VDC / 20 A. For G5PZ-X, this type of use can be recommended.(See individual datasheet for details)

G5PZ-X connection example

Each relay can interrupt 200 VDC each, up to 400 VDC in total.


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image Cautions for series connection

If the reset times of relays in series connection vary, or if one relay is damaged (welded), arcs may momentarily concentrate on one relay, leading to its failure or damage.

In the ideal state , relays (1) and (2) operate simultaneously and 1/2 the system voltage is applied to (1) and (2)

If relay (1) operates before relay (2) , an arc occurs with full voltage applied to (1) V(1) > V(2)


image Shunt current when relays are connected in parallel

A larger current may be energized by connecting relays in parallel. For example, two relays that can energize 200 A may be connected in parallel to energize 400

However, OMRON does not recommend parallel use.

In the ideal state , relays (1) and (2) operate simultaneously and the currents flowing through (1) and (2) are the same.


image Cautions for parallel connection

If one relay fails and cannot be energized, more current will flow to the other relay, resulting in its failure. In addition, since the contact resistance of each relay is not exactly the same, current concentrates in the relay with the lowest contact resistance, leading to its failure as well.

To prevent this, it is necessary to use a relay with a rating that has a margin in excess of the circuit current, or install a current sensor or fuse in the current line of each relay to prevent overcurrent.

Also, when a load is interrupted, the interrupting arc is always concentrated in one of the relays, resulting in a high risk of failure. Thus, control is necessary to ensure that no load (0 A) is applied at the time of interruption, or that the current is kept within the rated current of one relay.

Note that OMRON does not guarantee nor recommend the operation of this countermeasure example.

In the ideal state, relays (1) and (2) operate simultaneously, and the currents flowing through (1) and (2) are the same.
There is, however, a risk of current being concentrated in one relay due to a failure of one of the relays or due to differences in contact resistance.


:sparkles: Countermeasure examples

1. Select a relay with ample current capacity

2. Insert resistor for current balance


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Easy-to-understand explanations of the unknowns when using high-capacity power relays

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