Slow-action vs Snap-action Safety Interlock Switches...

Eric Nelson

Lifetime Supporting Member + Moderator
Join Date
Apr 2002
Location
Randolph, NJ
Posts
4,346
I have a customer using a tongue-operated interlock switch, directly connected to the safety inputs (SC, S1, S2) on an FR-D720 Mitsubishi inverter (no interposing safety relay). The switch is one of the cheapo ones from Automation Direct (LINK). The problem they are having is that, when they open the safety switch slowly, one of the contacts opens before the other, and this delay between the circuits opening faults the drive. This is the correct operation of the drive. The switch is the culprit here.

Automation Direct lists these switches as "slow action break before make". This switch has two normally-closed contacts, so I don't understand how this terminology applies. I always thought 'break before make' only applied to 'double-throw' contacts, like shown on page 3 of THIS PDF.

I suspect that the switch they're using is just cheaply made (what do you expect for $15?), but before I recommend a different switch, I figured I'd get some input here. I looked at other brands, and noticed that 'slow-action' seems to be the norm. The Omron D4NS series offers 'snap-action', but only on the 1NC/1NO versions (where I feel 'make before break' actually applies).

I guess my question is, should you be able to easily 'tease' the contacts on a safety switch? I have an Omron D4NS-3AF here, and although it is not a 'snap-action' model, there is a distinctive 'click', even when you insert/remove the key very slowly. I would expect that both circuits probably open/close damn near simultaneously. I don't have one of the AD switches here, so I don't know if they lack the 'click' action... :confused:

Any thoughts or recommendations? I would install a standard safety relay between the switch and drive, but this customer is looking for an inexpensive (read: cheap) solution... :rolleyes:

For reference, the fault they are getting is 'E.SAF', meaning only one of the safety contacts opened. In order to clear this fault, they have to press the STOP/RESET button on the front of the drive, which requires going into the control panel. It does not automatically reset. On standard safety relays I've used, when this fault occurs, the relay will automatically reset once BOTH contacts are opened and re-closed. Maybe there is a setting in the drive that will allow this fault to auto-reset? I'll have to investigate the drive manual.

🍻

-Eric
 
I would do nothing. Safety on the cheap is a liability waiting to happen. And when it does, lawyers will put everyone involved under a microscope. Just defending your actions could cost you everything. just my two cents worth.
 

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