# Using relay contacts in series to switch DC loads

We are using relay DRM570110LT and We need to switch 3A @220VDC in the output when we have a DC load. As the datasheet for this relay says, the rated switching current is 5 A, but I've been told this rated switching current is only for AC voltages. For DC voltages, the possible switching current is significantly lower (as per attached photo) and this relay can not be used for switching 3 A @ 220VDC in the output. Searching on the web, I found some solutions like connecting relay contacts in series to increase the switching capacity. But;

1- How could series-connected relay contacts result in increasing the DC current switching capacity?

2- How many contacts should I connect in series to be able to switch 3 A for a DC load? Does a general rule exist?

• Looking at that chart, you'll see that the DC capacity is inversely proportional to the DC voltage, but you don't say what your DC voltage is, assuming it's to the right of the curve: In general parallel-ing or serializing (higher voltage) relays to "share the load" is in general a bad idea- relay contacts in such situations have no way to synchronize themselves, so one contact will bear the brunt of the overload- reliability will not be as good as a properly rated relay. If you use multiple contacts on the same relay you might get away with it (for a while). Oct 21, 2018 at 14:59
• BTW, for higher current you would place the contacts in parallel, serializing helps with over-voltage, neither one is good practice IMO. If you could post the serial for higher current reference, I'd be interested in seeing that. Oct 21, 2018 at 15:02
• Thank you @isdi. I added the switching voltage to my question. Actually my customer is saying that it is not possible to switch 22VDC,3A using this relay. I wanted to know if there are any solutions to reach this braking capacity without buying another relay(like serializing the contacts,...) and if yes, how should I know the number of contacts to serialize to reach this DC breaking capacity? Oct 22, 2018 at 6:12
• Honestly, you should be using a definite purpose DC contactor for that voltage and current. A DC Contactor has magnetic blowouts to extinguish the arc when opening, and will be much more reliable then any simple relay. Oct 22, 2018 at 10:50

You need to find a relay rated for 220VDC. If it doesn't meet the DC voltage specification, you will draw an arc every time it opens which will quickly melt the contacts. Can you use a MOSFET or IGBT as a relay? How about a SSR (solid state relay)?

Using relays in series only increases the resistance of the relay when open.
It can also serve as a safety feature, would one of the relays fail.

It does not increase the switching capability since:
- you cannot guarantee they open at the same time.
- the rating is based on the short contact resistance increase during disconnect. This causes heat (ohms law) and will weld/damage the contacts upon close/opening action.

Your only option is to buy a relay rated for 3A 220VDC. Keep in mind that you need to follow the derating specifications when your load is not a pure resistor.

There are some options to help the situation, like replacing the flyback diode with as high a voltage as the relay can stand to speed up the opening and over-driving the closing voltage for a short time.