Estop Category for frequency converter

[hooze]

We sell ac-dc-ac power supplies with IGBT outputs that feed seires or parallel tank circuits. I normally connect my safety relay to the main breaker UVR. WHen the UVR drops the mains, the logic and gate drive system continue to fire normally until the DC bus voltage is dissapated and the tank circuit energy has been depleted.

Question: Would this be considered a controlled shutdown because the outputs fire after the breaker opens or is it still a Category Zero because I pulled the mains power?

 

[pfalcon]

Disclamer for the NFPA79: No direct legal requirement for following the NPFA79 exists to my knowledge. Through letters of interpretation OSHA has stated that failure to use a "Nationally Recognized Consensus Standard" for recognized hazards is cause for action. To my knowledge the NFPA79 is the only standard meeting OSHA requirements for its scope (Electrical Standard for Industrial Machinery). This creates and indirect regulatory requirement for the NFPA79 through the OSHA General Duty Clause. IMO.

(1) Category 0 is an uncontrolled stop by immediately removing power to the machine actuators.
(2) Category 1 is a controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved.
(3) Category 2 is a controlled stop with power left available to the machine actuators.

As I'm understanding, your machine actuators remain powered until the energy drains. Technically it doesn't qualify for any of these.

Your description is an uncontrolled stop. No circuitry is being used to actually create a stop condition therefore it's without control. Therefore not (1) or (2).
Power is not being removed immediately from the actuators therefore it's not (0) either.

Each machine shall be equipped with a Category 0 stop.

You need to kick in a line drain or break the conductors so that power to the actuator stops immediately for (0).

Each machine must have a (0). Estops can be either (0) or (1). Machines can have additional stops.

 

[petersonra]

We sell ac-dc-ac power supplies with IGBT outputs that feed seires or parallel tank circuits. I normally connect my safety relay to the main breaker UVR. WHen the UVR drops the mains, the logic and gate drive system continue to fire normally until the DC bus voltage is dissapated and the tank circuit energy has been depleted.

Question: Would this be considered a controlled shutdown because the outputs fire after the breaker opens or is it still a Category Zero because I pulled the mains power?

I don't think it is a category anything at all. It does not sound like this is part of a machine that would be subject to NFPA79. Is there any motion at all associated with the tank circuits these things are feeding? read carefully what a category 0,1,2 stop means.

 

[renosteinke]

Hooze might not have explained the application well enough.

If it's anything like the similar equipment I deal with, hitting the 'stop' button does actually stop whatever it is that the drive is doing. Right then. Completely.

What does not change is that the capacitors within still have a full charge. There's an internal circuit that 'bleeds' off this charge when power is lost. This can take 30 seconds or so.

Like an old TV set, you can have the dang thing completely 'unplugged' and still get zapped to heavens' gate ... until those caps bleed down.

It should also be noted that the capacitors are holding voltages way above the line voltages, usually over 1000 volts.

 

[pfalcon]

... Is there any motion at all associated with the tank circuits these things are feeding? ...

... If it's anything like the similar equipment I deal with, hitting the 'stop' button does actually stop whatever it is that the drive is doing. Right then. Completely.

What does not change is that the capacitors within still have a full charge. There's an internal circuit that 'bleeds' off this charge when power is lost. This can take 30 seconds or so. ...

NFPA79 doesn't address latent charge. The following doesn't actually apply to caps for example. It's intended for interlocks, line-side equipment such as work lighting, and batteries.

The color ORANGE shall be used to identify ungrounded conductors that remain energized when the main supply circuit disconnecting means is in the off position. This color identification shall be strictly reserved for this application only.

Having said that, a standard industrial practice is to wall off circuits and wiring that may hold latent charges. Disassembly of guarding required. Often a charge indicator is added to display when it's safe to open. Until then you're still considered to be working on an energized circuit (70E).

As Bob (petersonra) is pointing out, stop categories are intented to address motion.