I have a situation between myself (AC Contractor) and the fire alarm contractor. Here is the story. I have a 10 Ton split system with addressible duct mounted smoke detectors in both the supply and return duct work. The FA contractor has these pan relays that are used to break our 24v control circuit to kill the tstat and ultimately stop the air handler upon a signal from the system. Th ey are saying that my low voltage is burning up the relays. Our low voltage is fed from a 120/24 75VA transformer and I am told the pan relays are good for 3 amps. Our actual primary voltage is about 212v and secondary side is ~26v. Am I crazy or is this not possible. I cannot see that we can b drawing in excess of 3 amps, not to mention, the digital programmable thermostats probable cannot handle 3 amps. Any suggestions?
Air Conditioner Low Voltage Question
- Thread starter acguy
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I guess it could depend on how sensitive their equipment is. Assuming the system they have is 24v then putting 26V to it may be an issue. That is about a 9% increase in voltage.
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Check the amperage on the LV circuit as it goes through the FA relays.
Meg the LV wiring to make sure there is not a degraded conductor.
If you cannot find the problem, can you add a relay in the circuit feeding your LV transformer that the FA system can control? (would that even work?)
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
The way this is worded does not seem possible. A 120/24 transformer being fed by 212V and only putting out 26V?
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
I would check the amps on the 24v side and see what the actual amp draw is. Not sure what a pan relay is. If your actual voltage is 212v are you on a 208 system? Is the transformer set for 208v or 240v?
It is 208 but actual is 212 or so. The transformer is 120 v but only using one leg. Sorry i did not clearly state that. Th eonly thing thi srelay does is drop out the LV control circuit to kill the AHU on signal. It breakes the hot leg off the transformer. Their relay is is 24v as well but I would imagine that since they are using the same primary voltage as us, that their LV woul dbe simalar in voltage. Th eclaim is that we we are exceedingth the 3 amp rating thru our circuit which in turn burns up their relay. I just don't see how we can be exceeding that.
- Location
- Illinois
- Occupation
- retired electrician
The part that normally burns up on a relay is the coil. The coil should be supplied by the fire alarm system. What exactly is happening to the relays? If it is not contact failure, the problem is on the fire alarm side and is the fire alarm contractors problem.
MichaelGP3
Senior Member
- Location
- San Francisco bay area
- Occupation
- Fire Alarm Technician
Don, regarding addressable fire alarm system relays, I've seen contact failures outnumbering coil failures at least 2 or 3 to 1. And if it's a contact failure, the problem could still be on the FA contractor's side. I'm guessing that the OP isn't talking about a pan relay, but a PAM relay, of which there are different models, and not all of them are rated for (or are listed for use with) a 24 volt AC input. Can a link be provided by acguy to the exact make and model of relay that is being used? Also, are we clear that there are no infrequent intermittent grounds showing up as this unit runs (and vibrates)?
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- Location
- Illinois
- Occupation
- retired electrician
Michael,
If that is the case, then I would really expect that the fire alarm relays are being used outside of their contract rating or that they are cheap imported junk like so many things we see today.
MichaelGP3
Senior Member
- Location
- San Francisco bay area
- Occupation
- Fire Alarm Technician
Don: I should take a step back from my statement to clarify that I'm talking about all of the relays I've seen fail, most of which were attributable to having a ground fault condition in the field wiring downstream of the contacts pull high current, toasting the contacts. In normal service with clean wiring, more relay failures would occur due to the coil, or an asociated current limiting resistor breaking down over time (and heat).
Lemme ask is this right: they are PAM relays and the fire alarm system switches their state and opens removing 24 VAC from the Tstat to shut down the RTU? (kinda odd that the tstat is what is broken, normally its something else, but Im not a HVAC guy)
If this is the case, I have never heard of the switched voltage being an issue, unless there is a ground fault.
I would confirm that the PAM relay is supplied with the correct voltage (and its the correct rated PAM relay,most are rated at 10A Form C). FYI: most PAM relays are fire alarm rated see: http://www.1sae.com/products/bldgcontrol/pam/index.asp and http://www.firelite.com/datasheets/df-52068.pdf Even if it isn't, check to make sure the coil voltage is from a regulated source.
Oh and megger the conductors, yours and then theirs as suggested.
Oh and recheck everything, Its almost certainly not the fact you have 208V to 24 VAC transformer. There are 1000's RTU's that are 480/277V with a 277V to 24 VAC transformer. Never heard that excuse with those, but there is always are first.
If this is the case, I have never heard of the switched voltage being an issue, unless there is a ground fault.
I would confirm that the PAM relay is supplied with the correct voltage (and its the correct rated PAM relay,most are rated at 10A Form C). FYI: most PAM relays are fire alarm rated see: http://www.1sae.com/products/bldgcontrol/pam/index.asp and http://www.firelite.com/datasheets/df-52068.pdf Even if it isn't, check to make sure the coil voltage is from a regulated source.
Oh and megger the conductors, yours and then theirs as suggested.
Oh and recheck everything, Its almost certainly not the fact you have 208V to 24 VAC transformer. There are 1000's RTU's that are 480/277V with a 277V to 24 VAC transformer. Never heard that excuse with those, but there is always are first.
Thanks for your responses. They are PAM relays and I am not sure what their ratings are. All I know is the FA contractor mentioned they were good for 3 amps. My control voltage is running thru the contact side while the FA is the coil side. This particular transformer is 75 VA and controls (2) compressor contactors, (1) ID fan motor contactor and (1) tstat. In this case, they are not robbing 24v to powere the smoke detectors but have their own source. The failures, I am told, are on th econtact side of their relay. What I am having trouble undderstanding, is how can just those items powered by the transformer pull greater than 3 amps? In my opinion, no matter what we find, I don't thinks its my problem. They apparently need to upsize their relay to handle the load. Also, i was look thru the tstat lierature for the electrical ratings. the running current values range form max 0.6 to 1.0 amps, depending on which terminal is energizing, ie 1st stage cooling, 2nd stage cooling, etc. I have a meeting with the GC, FA contractor and the municipality on monday and just wanted some more opinions on the issue. Thanks again Guys.
MAK
Senior Member
- Location
- Massachusetts-Boston
I thought most electronics had a + or - of 10%. It would seem odd to me that a relay would be sensitive to over voltage of 9%.
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
I agree. Most likely the FA company uses a PAM 3 instead of a PAM 1,2 or 4. PAM 3 are generally used as EOLR's
fishin' electrician
Senior Member
- Location
- Connecticut
I break one side of the control transformer output so that the fans shut down immediately, breaking power to the stat allows the fans to continue running until they time out, but I'm not an HVAC guy either.
Mgraw
Senior Member
- Location
- Opelousas, Louisiana
- Occupation
- Electrician
Most commercial A/C units have instant on, instant off fan relays or contactors. Alot of residential units have delay on, delay off relays or circuit boards. The best place to break the control circuit is at the transformer. Many commercial unit have the transformer in the condensing unit so breaking the circuit before the T-stat is the next best option. You could just break the fan control circuit but then the condensing unit would continue to operate and could damage the compressor.
Security101
Senior Member
- Location
- Northern Indiana
esobocinski
Member
- Location
- Ann Arbor, MI
Let me back up and make sure I understand: Transformer is rated 120v in, 24v out, 75 VA. But you're driving the input at 212v and using a secondary tap to get an acceptable output voltage. Is the transformer rated for higher input voltages (e.g. as 208/42 or 240/48)? If not, that would be a code violation, right?
I expect that doesn't make a practical difference, since transformer insulation is capable of much higher voltage than the transformer rating. I also expect that the 75 VA rating doesn't matter to the failing relays, because the actual draw on the secondary is likely governed by the load (thermostat or contactor). However, the fact that the 75 VA rating was emphasized in the original post has me worried. You aren't counting on the transformer output as a current limiter, are you? If so, that would be a problem: The theoretical current limit of a transformer is governed by the impedance of the windings. If you're overdriving the windings with a higher input voltage than rated, then the transformer is capable of a higher output current and power than its rating. In other words, it's capable of delivering more than its rated 75 VA (until it overheats and fails from the overload).
I expect that doesn't make a practical difference, since transformer insulation is capable of much higher voltage than the transformer rating. I also expect that the 75 VA rating doesn't matter to the failing relays, because the actual draw on the secondary is likely governed by the load (thermostat or contactor). However, the fact that the 75 VA rating was emphasized in the original post has me worried. You aren't counting on the transformer output as a current limiter, are you? If so, that would be a problem: The theoretical current limit of a transformer is governed by the impedance of the windings. If you're overdriving the windings with a higher input voltage than rated, then the transformer is capable of a higher output current and power than its rating. In other words, it's capable of delivering more than its rated 75 VA (until it overheats and fails from the overload).
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