Intermittent contactor operation

Status
Not open for further replies.
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Hey, all. I am troubleshooting a commercial kitchen fire-suppression system where a contactor does not function correctly some of the time. We replaced the micro-switches in the control head, and the operation is still intermittent. Sometimes, the gas solenoid valve drops out upon tripping, and sometimes it doesn't.

I don't know yet whether the offending conrtactor is normally energized or deenergized, but I don't believe the issue is actually electrical. Is there a suitable lubricant made for a sticky contactor armature that can be applied without disassembly of the contactor? I hope to resolve the problem tomorrow. Thank you, Larry
 
ptonsparky said:
WD40. I would sure look for bugs.
Click to expand...

Just FYI, WD-40 is NOT a lubricant. The WD literally stands for Water Displacer. The 40 is because it was the 40th formulation of the product during development.

A tiny puff of graphite on the friction surface should be enough. If not, it’s probably time to replace the contactor.
 
The makers of WD-40 now also make an actual silicone based lubricant spray too. AFAIK that one is not intended for water displacement use.

Sent from my XT1585 using Tapatalk
 
OldSparks said:
Just FYI, WD-40 is NOT a lubricant. The WD literally stands for Water Displacer. The 40 is because it was the 40th formulation of the product during development.

A tiny puff of graphite on the friction surface should be enough. If not, it’s probably time to replace the contactor.
Click to expand...
Graphite being conductive, I would be very careful where I puffed it. (Although like the case of a stream of water droplets, the conductivity between graphite particles may be pretty low.)

Sent from my XT1585 using Tapatalk
 
180223-2143 EST

Remove all power. Manually move the armature. Is it free to move?

Disconnect the wires to the contactor coil. Connect the coil to a Variac. You want a switch in the circuit.

Apply 120 V (or whatever is the nominal voltage). When power is applied the contactor should have a solid clunk.

Reduce voltage to dropout and note the dropout voltage and whether there is a good sound. Don't leave powered at the dropout. 50 to 60 V might be good dropout.

At various voltages see where the contactor pulls-in. I would like to see 70 V or so on a 120 V coil.

.
 
Larry,

I would suggest that it is likely the gas valve itself that is sticking. Fire shutdown systems should be designed as fail safe. Meaning power should be required to hold contactors and solenoid coils on, no power, then no gas or electric supplied to the hood equipment. Every time power goes down the gas should shut off, and the pilots need to be relit. If the system is powered down or tripped the valve should close with no power applied to it.

The fire industry uses a small enclosed relay, called a "reset relay", with a reset button and pilot light on the cover to latch the relay on. Momentary power outages should unlatch the relay and lock off all power and gas, until the relay is reset and pilots relit. Similar to a 3 wire motor control station.

Solenoid valves are pilot operated, meaning that there are pilot orifices in the valve design, that use line pressure to open and close the valve. The solenoid coil operates a plunger that opens and closes the pilot ports to get the valve to actuate, by line pressure. If any particulate from the gas line gets into the plunger area or the pilot ports the valve will not seal correctly. The diaphragms in the valve can become sticky or ruptured with age as well.

The fire contractors in my area have switched to mechanically cable operated valves, in place of electrically operated valves for this reason. Mechanical valves are spring closed, and tripped mechanically by the steel cable that runs through the melting fire links and the manual pull stations. Its a cheaper valve, and less prone to failure. Also does away with the need to relight pilots, when there is a power outage.

If the gas solenoid is the one sticking, it should be replaced. No lubricant should ever be used on a gas valve or a contactor, it will cause problems later on. Clean and dry, is what it takes for reliable operation.

MTW
 
After my latest visit, it appears the problem is mechanical, not electrical. The contactors appear to operate every time the micro-switch levers are manually operated, but not every time the suppression head is physically tripped. The head is an older type that looks like this one, except that there are three* micro-switches to the right of the mechanism:

FSH.jpg

The sliding mechanism doesn't always move fully to the far right, even after I spritzed the moving parts with WD-40. I will meet the fire-suppression guys tomorrow morning to make sure everything is cleared up. The fire marshal closed the restaurant Friday morning because of repeated non-compliances, and they really want to re-open.

This system does use the electrically-operated gas valve with the above-mentioned reset-relay control box, plenty of which I have wired in the past. I knew the control box was not the issue, because the exhaust fans and gas shut-down either all worked or failed to work at the same time, so we knew the problem was in the control head.

*Only two switches were originally wired: one that turned on the fans and shut down the gas, electrical, and hood lighting, and one that operated the fire alarm panel via low-voltage wiring. I actually had to use the third to add shut-down of the make-up air blower's contactor, as that had not been done when the system was originally installed.
 
I was going to also ask about gas valve since that was only thing you mentioned in OP that seemed to not operate as expected. Is it an electric solenoid or a mechanically operated valve? If you think you have mechanical problems then mechanical valve probably has problems as a result.
 
I just went back this morning, as the operation was still intermittent. It turns out that there must have been some sort of electrical "feedback" for lack of a better term. Sometimes, the gas reset box would drop out as expected, sometimes not, and sometimes it would not drop out until the micro-switches were released.

I had to rewire last visit so the make-up air blower would shut down upon system trip, as it had never been wired to do so. Today, I rewired so the gas valve was powered by the same wire as the make-up air contactor, since they both should lose power upon system trip. That's how I often wire it when I wire new systems.
 
LarryFine said:
I just went back this morning, as the operation was still intermittent. It turns out that there must have been some sort of electrical "feedback" for lack of a better term. Sometimes, the gas reset box would drop out as expected, sometimes not, and sometimes it would not drop out until the micro-switches were released.

I had to rewire last visit so the make-up air blower would shut down upon system trip, as it had never been wired to do so. Today, I rewired so the gas valve was powered by the same wire as the make-up air contactor, since they both should lose power upon system trip. That's how I often wire it when I wire new systems.
Click to expand...
Like 120 and 240 volt loads controlled by same contact? That will get you into trouble almost every time if you don't somehow isolate them or use two pole switching devices.
 
Status
Not open for further replies.
Top