safety relay and safe torque off
Close
Login to Your Account
Results 1 to 4 of 4
  1. #1
    Join Date
    Oct 2014
    Country
    UNITED STATES
    State/Province
    Maine
    Posts
    795
    Post Thanks / Like
    Likes (Given)
    71
    Likes (Received)
    379

    Default safety relay and safe torque off

    I'm looking at a simple system consisting of a VFD with safe torque off (probably going to be a Nidec Control Techniques Unidrive M300) and one e-stop button, both running dual channels. As I understand it, standard practice would be to connect the e-stop button to an e-stop relay, and the outputs of the e-stop relay to the STO inputs of the drive.

    Assuming I'm using a basic safety relay, not one of the fancy ones with cross circuit detection, pulsed outputs, or sequence monitoring, what does the e-stop relay add safety-wise?

    With a safety relay, I hit the e-stop button. No single failure prevents it from cutting power to the safety relay on at least one channel. When that happens, power to the STO inputs is cut and we get a category 0 stop. To restart, release the e-stop button and press reset.

    Without a safety relay, I hit the e-stop button and it cuts power to the STO inputs. To restart, release the e-stop button and press reset.

    The main difference I see is whether the reset button is connected to the safety relay or to the drive, both of these would appear to meet the requirement that releasing the e-stop button doesn't restart the motion, but merely permits the motion to restart. Is there a significant difference between the reset circuits behind the scenes?

  2. #2
    Join Date
    Dec 2003
    Country
    UNITED STATES
    State/Province
    Washington
    Posts
    4,633
    Post Thanks / Like
    Likes (Given)
    2134
    Likes (Received)
    3922

    Default

    Is this something you’re integrating for yourself, or for a customer?

    As long as the E stop button is dual channel, and forced to open contact blocks, you should be fine without any kind of relay at all.

    You really need to do a HIRA and make some kind a determination about the inertia of the rotating load and whether or not somebody can be injured as it coasts to a stop.

  3. Likes DanielG liked this post
  4. #3
    Join Date
    Mar 2011
    Location
    Denver, CO USA
    Posts
    10,214
    Post Thanks / Like
    Likes (Given)
    58
    Likes (Received)
    5695

    Default

    Your analysis is correct.
    At this point is becomes the engineers responsibility to decide what level you need based on the specific application and hazards.

  5. Likes DanielG liked this post
  6. #4
    Join Date
    Oct 2014
    Country
    UNITED STATES
    State/Province
    Maine
    Posts
    795
    Post Thanks / Like
    Likes (Given)
    71
    Likes (Received)
    379

    Default

    Thanks. I've never had a system not have a safety relay before. This system may well end up having one as well, but the question came to me as I was reading the STO documentation. Prior systems have always had a safety contactor, so this never came up.


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •