E-stop should not be used for LOTO, that's what disconnects are for. Scenerydriver mentions safe torque off (STO) and while the STO circuit will safely remove power to the motor, it will not isolate power to the drive, so you would be safe from the motion hazard but not necessarily the electrical, depending on what part of the system you're working on.I like to think when I hit the e-stop and put my lock on it, powered circuits are disabled.
The STO inputs on VFDs physically disconnect the drive circuitry from the IGBT output transistors. IF STO is off, there can be no power to the motor, and thus no motion. You are not electrically isolated though so beware, depending on what part of the machine you're working on.Thank you SceneryDriver.
There's no obvious hazard, but in my experience e-stop circuits should kill power to plc outputs and anything that controls motion.
What you say about killing power to drives makes sense but I've also have seen my share of runaway drives.
I like to think when I hit the e-stop and put my lock on it, powered circuits are disabled.
It may not ALWAYS be the case. For instance, there is no reason to remove power from PLC outputs that only energize indicator lights. In fact, there may well be very good reasons not to do so.Again, I get that emergency circuits can't always be trusted but I aways thought power supplying plc outputs should be removed provided it doesn't cause an additional hazard.
I guess that's not the case.
Let’s be clear here. Assuming this is US jurisdiction (others may be different) there are quite literally over a dozen “LOTO” regulations put out by OSHA. Most people are familiar with 1910.147 and a lot assume that this is THE LOTO regulation but it’s not. This section requires that you physically disconnect or discharge all energy sources. Disconnecting control power is not enough. The reason is for instance I could (stupidly) jumper out your lock. I have todo this sometimes for electrical testing and it’s far easier to disable controls than most people realize.I understand e-stops can't be fully trusted for loto, but it's common practice in many circumstances, at least where I'm at.
In fact Most e-stop buttons on machinery here are designed with a loop to accept a hasp and lock.
If I had to position myself in a potentially dangerous situation, I would certainly turn off the main disconnect or at least isolate power nearby. But honestly, for many simple tasks like adjusting a switch, it's often not feasible to shut off the main disconnect.
To recover some machines from a full power down can often take considerable time with the risk of damaging (as you mentioned) VFDs of other electronics.
Again, I get that emergency circuits can't always be trusted but I aways thought power supplying plc outputs should be removed provided it doesn't cause an additional hazard.
I guess that's not the case.
It's perfectly acceptable to open the neutral in a disconnect, as long as all phase conductors are simultaneously opened as well. It is in fact a requirement to do so for power that serves certain devices; gas station fuel pumps, for instance.Thank you for shedding light on the subject.
I've been at this nearly 30 years myself and never fully understood some of the rules regarding it.
Reading the various and varying safety documents can be confusing and usually quite boring.
To add to the confusion, Machinery built and appoved to EU standards is often signed off and deemed compliant here in USA dispite having different rules on safety. For instance disconnecting neutral in a disconnect.
As for emergency circuits, following JIC and Nfpa79 rules over the years I've come to expect seeing a master relay that will drop out any motion devices.
More recently they have safety relays (like Pills) to take their place but they act the same way.
My original question question was regarding a machine the leaves power to pneumatic valves after estop is activated (that doesn't pose a threat to de-energize).
So I guess the short answer is It's allowed.
But to me that's a poor design even if the air is disconnected to machine.
Doesn't it seem counter intuitive for manufacturers to make e-stop buttons with a mechanical slide to accept a lock if it isn't allowed for Loto?