I'm trying to find reliable information regarding the duration of inrush current for high pressure sodium lamps with electronic ballasts.

This document states that: "In reality, due to the wiring impedances, the inrush current for an assembly of lamps is much lower than these values, in the order of 5 to 10In for less than 5 ms."

However it is not specific to high pressure sodium lamps.

The lamp I wanted to install have inrush current rating <30A, but there is no duration indicated. The retailer of the contactors I intend to use needs the duration (in microseconds) in order to recommend a specific contactor.

How do I get the duration?

  • \$\begingroup\$ Why not test it? \$\endgroup\$
    – Andy aka
    Commented Jan 29, 2021 at 12:11
  • \$\begingroup\$ @Andyaka I did already test this, unfortunately a lot of contactors were damaged after 2-3 months, meaning that some poles (out of 4 per contactor) stopped working. Because unlike a switch which switches off when overloaded the contactors are controlled by a control current, and there is no apparent damage if the inrush current is too high. That's why I am aiming for a more professional approach \$\endgroup\$ Commented Jan 29, 2021 at 12:23
  • \$\begingroup\$ Test the inrush current value using a current probe and an oscilloscope. Test a few to get a feel for the spread. If necessary use an NTC thermistor in series to reduce the inrush level. \$\endgroup\$
    – Andy aka
    Commented Jan 29, 2021 at 12:25
  • \$\begingroup\$ Ask the manufacturer? \$\endgroup\$
    – Kartman
    Commented Jan 29, 2021 at 12:55

1 Answer 1


Contactors are rated according to duty. Manufacturers tend to quote the highest figure the contactor can switch which tends to be AC1 or AC2. This is resistive. As you move towards inductive switching such as motors and SON lights etc you get a high inrush current, 10 to 15 times.

Contactors should be selected from a higher range such as AC3 or AC4. The contactor are larger, more robust and much more able to dissipate the heat during switching operations. A similar situation exists for LED lighting. On a inductive circuit such as a SON lamp you are looking at split seconds of over current, the time it takes to magnetize things. Clearly as the magnetism starts to build so the current starts to drop. On an electronic ballast the front end of the circuit uses capacitors and the inrush current can be substantially greater, up to 50 x.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.