I have used the following arrangement for switching a solar pump:

Input from 3 solar panels (40V/330W each)--> relay switch (S-KAP S90-S-DC05V, 30A 250VAC/30VDC)--> solar pump (8.4A, 30-300VDC, 1.4kW)

Everything starts fine, but after some time the relay burns out.

Any ideas why this is happening? Any help would be appreciated.

  • 1
    \$\begingroup\$ Please explain "relay burns out". Do you mean the coil heats up to the point where the wire burns or do you mean the contacts burn and no longer function? Or perhaps something else? \$\endgroup\$
    – jwh20
    Mar 1, 2022 at 14:43
  • 3
    \$\begingroup\$ Your relay is rated in bold 30VDC, So P = 30Ax30V = 900W You have 3 solar panels of 40V and 330W. 330x3 = 990W So you exceed in voltage and in power at max rate \$\endgroup\$
    – Mat
    Mar 1, 2022 at 14:50
  • 2
    \$\begingroup\$ NB: switching DC is really difficult ... What kind of pump? DC ... inductive ? \$\endgroup\$
    – Antonio51
    Mar 1, 2022 at 14:52
  • 1
    \$\begingroup\$ Please link to relay datasheet that you bought. Most likely it indicates you are exceeding the ratings. What is printed on the relay does not mean it can switch 30A and 30VDC simultaneously, and you already habe 40V panels. \$\endgroup\$
    – Justme
    Mar 1, 2022 at 14:53
  • 5
    \$\begingroup\$ Are your panels in series or parallel? \$\endgroup\$
    – winny
    Mar 1, 2022 at 14:59

2 Answers 2


Typically contacts rated for AC and DC, whether operated by hand (switches) or a coil (a relay) will be rated for a much lower DC voltage. This is because an AC arc at the time of contact opening will tend to self quench when the AC crosses zero. However a DC arc will persist much longer (or indefinitely!) and is damaging to the contacts. There is a "DC Rule of Thumb" for switches that list an AC voltage rating only, which is that the highest AC current quoted for the switch should perform satisfactorily up to 30 volts DC. Your relay sounds like one of these.

Many people, used to AC power, don't realise how brutal DC can be, even at lower voltages. There have been many cases of destructive fires caused by using AC type components and techniques (switches, connections, cable terminations, etc) on DC solar installations. Careful study of this topic may save much heartache later. People think of batteries and say "it's only DC". In the early days of DC mains power, switch operators in power houses were sometimes provided with long handled wooden paddles to try to break the arcs formed when opening the bare knife switches, which dripped molten copper and emitted blinding light. If it didn't work, I think they probably ran for it.

AC switch 240v 10A rated, used to break 240v 7A DC enter image description here

  • \$\begingroup\$ I understand DC switching is more complicated, is there any way we can switch such HVDC using DC contactor or Solid state devices like SSR or MOSFET's, could you please hep me guiding the same. \$\endgroup\$ Mar 2, 2022 at 7:02
  • \$\begingroup\$ Is it a good idea to use Solid state devices instead of mechanical relay to handle such DC power? Could you please add light on this? \$\endgroup\$ Mar 4, 2022 at 5:41
  • 1
    \$\begingroup\$ Having personally experienced the failure of a contactor switching 50,000 amps at 600 volts DC, your description of dripping copper is a bit inaccurate; it was more of a molten copper spray, and jamming a wooden paddle into it would not have been a good idea: running away is the recommended option. \$\endgroup\$
    – jayben
    Mar 5, 2022 at 11:26
  • \$\begingroup\$ @jayben - By the early 1900s, coal-fired power units featured outputs in the 1 MW to 10 MW, so with a +500v - 0 - 500 v three wire system a plant might be feeding 1,000 to 10,000 amps, and I never said the wooden paddles were a very good idea ;-) I think it has always been a bad idea to use isolation switches to break load anyhow. Automatic breakers were in use from about 1904. \$\endgroup\$ Mar 5, 2022 at 14:36

The root cause of the failure is the unsuitability of the relay that was used to control the 120V DC, 8.4 A motor.

An AC3 duty motor contactor may be used for DC3 motor duty, by connecting its contacts in series.

The following data is for a 3 x 400V~ 10A AC3 contactor, on DC3 duty.

enter image description here

Here's another.

enter image description here

Either of them may be wired, as shown, for DC3 duty.

enter image description here

Based on such data provided in the data sheets, any reputed make of contactor may be used.

  • \$\begingroup\$ It is important to consider the inductance of the load, as it is the stored energy that causes an arc to strike across the opening contacts - a purely resistive load won't arc. Once arcing has started, it could fizzle on until the switching device is destroyed, so you need to choose one that can handle the inductance of your pump; DC motor contactors often have permanent magnet 'blow-outs' to lengthen and cool any arcing. \$\endgroup\$
    – jayben
    Mar 5, 2022 at 11:33
  • \$\begingroup\$ I do agree. However, exclusive DC motor contactors are not available for lower ratings such as this. \$\endgroup\$
    – vu2nan
    Mar 5, 2022 at 13:07

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