One of our customers gave us a sample product for inspection. It has a relay output for 220V loads (e.g. fan, bulb, etc.)

Just out of curiosity, what is the meaning of the \$\mu\$ symbol here?

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  • \$\begingroup\$ It would be more search friendly if your question title identifed the item (contactor, relay, etc.) rather than just "product". \$\endgroup\$
    – Transistor
    May 15, 2020 at 12:26

1 Answer 1


It appears to indicate “micro disconnection” with a physical contact gap that is not of a certain minimum size. From this Shrack/Tyco document:

Micro disconnection Adequate contact separation in at least one contact so as to provide functional security. Note: there is a requirement for the dielectric strength of the contact gap, but no dimensional requirements.

It may be described in IEC/EN 61058-1, but I do not have a copy to confirm.

  • 1
    \$\begingroup\$ BS EN 60730-1 has the same definition for micro-disconnection. You could contrast it with the definition of full-disconnection which is contact separation in all supply poles other than earth so as to provide the equivalent of basic insulation between the supply mains and those parts intended to be disconnected. There are electric strength and dimensional requirements for full-disconnection. \$\endgroup\$
    – Martin
    May 15, 2020 at 14:03

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