Electronic – Symbol or marking on safety relay


I found this relay inside an industrial x-ray machine. What does the rectangular box with a horizontal line through it, in the bottom left of the first picture, mean?

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Best Answer

As others have answered, the symbols are those of a relay and fuse.

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Figure 1. European electrical control drawing symbols. The first contact will be numbered 1x, the second 2x, etc. Normally closed will be numbered n1 - n2 (where n is the contact) and normally open will be numbered n3 - n4.

The Pilz PNOZ X2.8P is one of a range of "E-STOP relays, safety gate monitors" used in industrial machine control cabinets. Internally there may be three relays arranged in a configuration similar to that of Figure 2.

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Figure 2. Approximate internals of a dual-channel safety relay.

How it works:

  • The loads to be made safe in the event of an emergency stop or safety gate opening are connected to terminals 14, 24 and 34 with power fed in on 13, 23 and 33.
  • The safety reset button is wired to K1.
  • The 2-pole emergency stop button contacts are wired to K2 and K3.
  • On power on (to A1/A2) all relays are de-energised.
  • Pressing RESET will energise K1. Provided the e-stop contacts are closed, K1 contacts will energise and K2 and K3. These will then latch on by their own contacts and remain on when RESET is released.
  • The load will be powered by the series-connected contacts of K2 and K3.

Dual channel

These relays are self-monitoring for a single failure. e.g., let's examine what happens if a K2's contact on line 33-34 welds during opening.

  • E-stop is pressed. K2 and K3 are de-energised.
  • For some reason a normally open contact of K2 welds and fails to open. K3's contacts do open, however, and the power is removed from the circuit to be made safe.
  • The contacts of K1, K2 and K3 are "guided" type. i.e., they are in a "comb" which prevents the normally-closed contacts from making contact until all the N.O. contacts are separated.
  • As a result of the fault the N.C. contact of K2 below the coil of K1 (reset relay) will not close and the circuit can not be reset.

The relay also protects against shorts on the e-stop wiring:

  • If the left e-stop contact is shorted out then pressing the e-stop will only de-energise K3. K2 will remain latched on. K3 should disconnect the load.
  • The fault will be detected at reset. K1 will be unable to be energised as K2 has not dropped out.

Terminals 41 and 42 can be used as fault indication contacts.

This should be enough information to give some understanding of the inner workings of the safety relay.