An electrical design/controls question:
In new installations, we’ve monitored all motor disconnects (MCC as well as local) via auxiliary contacts for years now and programmed PLCs to turn motors off when a disconnect is off and prevent turning them on until given a start command after disconnect is back on. This avoids unexpected starts when turning on a disconnect (contributing factor to some fatalities) as well as allows the HMI to indicate exactly why the motor won’t run (for example: “MCC disconnect off” or “Local disconnect off”).
When integrating new equipment into existing plants, this turns into more of a pain than it seems it should be. Often existing disconnects don’t have auxiliary contacts, long and inconvenient cable runs are required, and the main MCC breaker (upstream power) is often not easily monitored (while not the most likely scenario, turning on upstream disconnect could potentially cause on unexpected start if not monitored). It all can be done, but it tends to throw a monkey-wrench into standardization. Getting local personnel to actually get the disconnect monitoring in place often takes more follow-up from engineering than any other part of the job! So we’re looking at using current sensing relays mounted in the MCC immediately after the starters instead of disconnect monitoring.
Simple install: one small item with two wires to add into MCC below each motor starter.
Economical: adding a conductor for $20 current switch to motor control cable we already run will cost less than getting a cable run for local disconnect monitoring, let alone replacing existing disconnects that don’t support auxiliary contacts.
Complete upstream & downstream monitoring: Prevents motor run (based on PLC programming) when 3-phase power is disconnected anywhere upstream or downstream of motor starter (for example, MCC supply and local disconnects at top and bottom of a bucket elevator).
Increased standardization: one input and associated fault does the job regardless of layout in plant where the system will be installed (otherwise, some would have one disconnect input for a motor, while others may have 2 or 3).
Less specific fault info: HMI can’t point to the specific disconnect that is off. For example, “No current flow - check MCC & local disconnects” instead of “MCC disconnect off”.
No fault until start attempted: The fault would be triggered when current was checked a few ms after start command rather than when disconnected was turned off.
Turns motor off when disconnected, but doesn’t prevent start attempts which allows very short window for disconnect to turn on motor: This would be like motor starter auxiliary contact monitoring; it doesn’t prevent starting the motor, it just turns it off again in a few ms if there is no current flow. We’re not trying to prevent starting a motor while disconnected (it won’t start–that’s what the disconnect is for). We do want to make sure it cannot be left on while disconnected so that it starts unexpectedly when disconnect is turned on. Using this method, a start attempt nearly simultaneous with disconnect re-engagement could result in disconnect turning on the equipment, though that risk is limited to a window the size of the necessary delay for current sensing (based on one switch, we’re looking at ~200 ms + ~ 20 ms for PLC response). This is nearly–but not quite, I think–equivalent to the risk of disconnect being turned back on and run command being issued nearly simultaneously using the disconnect monitoring method.
We’re thinking of monitoring only one of the 3 phases for current. However, it seems this should be as reliable as monitoring auxiliary contacts in disconnects, and probably more so in some cases (non-permanently attached auxiliary contacts, or more disconnect locations than are monitored).
The less specific and delayed HMI fault display looks like a reasonable trade-off for the benefits above as long as we’re not missing something that would reduce safety. The third con is more concerning, though still likely safer than not monitoring all disconnects. We’re aiming to increase safety–as well as the other benefits–and want everyone to go home to their families well every day. While I was not involved in the controls design, programming, or incident, I’ve been to the funeral of a friend nearing retirement (while I was just starting my career) who was killed in an incident that would have been prevented if the PLC had turned off the motor when the disconnect was off (there were other factors too). I doubt the new worker who was sent to turn on that local disconnect will ever forgive himself for what he saw in the next second, though he was just doing as instructed.
What do you think of monitoring current rather than disconnects? Anything to add to the comparison above? Other suggestions?