I selected a relay for my application by seeing the maximum and current rating of Relays but I am confused if the relays works fine with the given rating for all kind of loads. In my circuit the relay is being used in the primary side of the LLC resonant converter which is working under 40A rms and 400VAC(Load is primary side of transformer,inductive) The relay which I selected has rating of max 50A and 480VAC. I could not really understand the contacts rating for different loads. Im really confused it I can use this relay for my application. Please let me know if u have any leads to this. I attach the relay datasheet for your reference. Thanks in advance enter link description here

PS: I dont do live switching

  • \$\begingroup\$ Where does it say "voltage rating decreases" in that document? Don't paraphrase, quote verbatim if you want help. \$\endgroup\$
    – Andy aka
    Commented Dec 20, 2020 at 12:35
  • \$\begingroup\$ My bad. I edited it. I misunderstood by checking electrical life column in datasheet. Thanks for correcting \$\endgroup\$ Commented Dec 20, 2020 at 12:39

1 Answer 1


If you NEVER do live switching, you should not have any problems. The contact rating for different loads is presented only as a factor determining contact life as a number of switching operations. I would as interpret that as live switching operations.

Inductive load is an issue with live switching. At some level of voltage, current and power factor or L/R, there is a potential for failure to interrupt the current the first time live switching is attempted.

The linked data sheet gives no data for inductive loads except to the extent that, for contact life determination, some unknown level of inductance is assumed in the electronic ballast load. The life for a capacitive load is determined per a cited standard.

The title of the specification page says that the product is suitable for motor load, but that is not at all supported by the specification details.

Note that the UL recognized component ratings support 40A at 347 VAC resistive. Operation above 250 V is not supported by VDE. That should discourage operation 40A, 400V inductive circuit operation except for that fact that the use is stated as "no live switching."

Re Comments

Your comment "Please see the LLC converter diagram" raised the question "what diagram?" I did not see a diagram so I discovered that you had asked previous questions about this. After reviewing those, it seems to me that you are just trying to select the number of series inductors in the circuit for various tests. It seems to me that you should just wire them all in series providing connection points for a shorting conductor that could be connected as shown below. I would be inclined to connect the inductors by bolting ring lugs together or bolting ring lugs to the inductor terminals if the inductors have bolt connections. The bolts could be mounted on some kind of rigid insulating material. You would connect the shorting conductor at any point just by bolting it together with the other two. Alternatively, you could attach sockets for some kind of pun connector at each connection point.

enter image description here


The example shown below shows the bolted connection that I described above. The wire and the two lugs attached to wire are rated about 80 amps. When using two nuts as shown, a brass nut would be better between the top lug and the two lugs below. The red material is fiberglass channel. It is a good strong insulating material that can withstand a temperature significantly higher than would be expected with the wire carrying rated current. It can be drilled reasonably easily, but I didn't want to drill it for this photo. This is stuff I have in my workshop.

enter image description here

  • \$\begingroup\$ Hey Charles, I am really happy with your answer. What if I am using SPDT for the two lines having same rating? Most of the SPDT having rated as for ex: 50A(NO) and 20A(NC), In this case can I use the Relay with both the line rated as 40A? again No live switching case \$\endgroup\$ Commented Dec 21, 2020 at 13:31
  • \$\begingroup\$ If a contact is carrying current when it is opened or carries current as a result of closing, that is live switching. If the contact simply selects a future path for current that will flow as a result of some other switch closing, that is not live switching. If there is any possibility of a contact opening while carrying current, it must be rated to interrupt that current. Since a possible mode of failure of a relay is to unexpectedly change state, it is probably not a good idea to select a relay that can not open the current that it might be carrying. Safety regulations may restrict use. \$\endgroup\$
    – user80875
    Commented Dec 21, 2020 at 14:58
  • \$\begingroup\$ I am sorry I did not understand it fully. Lets say I want to switch between two lines using SPDT each line is rated for 40A. If i use a relay which is rated for 50A(NO) and 35(NC) will it work? I energies the relay(auxiliary power) and keep the relay to the position I need before turning on the main circuit. I just want to divert the power flow and not to change the position of the relay while the circuit is ON. In this case does it work? Actually this doubt is related to the above question I asked. I thought of using one relay for each line (SPST) because I could not find SPDT for my applica \$\endgroup\$ Commented Dec 21, 2020 at 15:36
  • \$\begingroup\$ Please let me know if I can use SPDT relay to divert power flow between two lines for the mentioned ratings. Or else I can use two relay for two line and make one relay open and one short to divert the power flow or vice versa. I also could not find any other way to tackle this switching problem. Please let me know \$\endgroup\$ Commented Dec 21, 2020 at 15:40
  • \$\begingroup\$ I did not realize that you want to select either of two sources for a load. I thought it was selecting which loads to connect to the source. To select either of two sources, you should use SPST to prevent the two sources from being connected together. It would be better to use a relay that is rated for live switching because of the risk of the relay switching accidentally while live. Using a mechanical SPDT switch that is not rated for live switching would be better, but still subject to operator error. \$\endgroup\$
    – user80875
    Commented Dec 21, 2020 at 18:01

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