What's the purpose of the form b in 4a1b relay?

Question

This relay AHER4191 has contact arrangement 4formA + 1formB. I know the 4formA (pin 9~12 pin 1~4) are for L1/L2/L3/N lines but don't quite understand in which scenario the 1formB (pin5, 8) may be used?

Answer 1

Form B is for a low current DC capable switching contacts. They have different material that makes it suitable for DC wet making and breaking that AC only contacts do not need (because if the zero crossing behavior). Look at how much lower the ratings are, 1A at 227 AC or 30V DC compared to the 50A of the 440V AC switching pins.

As such, some may be used for accessory loads that you may need separately from your main loads. But since it's NC in your relay, that's unlikely.

Specifically though, Panasonic says it's a feedback switch: An optional NC contact as feedback contact can handle low level loads down to 10mA / 5VDC. for wallbox or inverter use. And a mirror contact.

https://industry.panasonic.eu/products/components/relays/power-relays/he-r-power-relay

Based on IEC 60947-4-1 Low-voltage switchgear and controlgear – Part 4-1: Contactors and motor-starters – Electromechanical contactors and motor-starters section "Requirements for auxiliary contact linked with power contact (mirror contact)"

Answer 2

Typically two things: one, so it can latch on (requires NO); two, to read state of the remaining contacts (NO or NC will do). This is more obviously useful in big contactors, where you can't spare a main pole, but the little side contact helps out.