I haven’t come across the following application before, where integral face and bypass steam heating coils shall have auto-reset freezestats and latching relays. I need to provide latching relays with LED indicator of relay status for each freezestat (10 total) so that the relay will remain latched after the freezestat resets. A momentary reset switch will be provided for remote contact reset from the DDC system for resetting the freezestat alarm. The relays shall be unlatched after a power fail.
I was planning on using DPDT relays for the latching circuit and understand how to create the latch using the first set of N.O. contacts on the relay wired in parallel with the N.O. momentary switch to keep the relay coil energized. However I’m struggling with how to achieve the above mentioned circuit in the real world and am hoping someone has experience doing this and provide some insight.
I typically have multiple freezestats (SPDT) in series with each other and a 24VAC relay coil (with one set of contacts as an input to the DDC controller) so that when any one of the freezestats trips it creates an alarm through the BAS system. The freezestat acts independently of the DDC system via a hardwire interlock and shuts down the system.
It sounds like you have the right idea. Your N.O. freezestat will energize the relay (and LED) when it trips. One set of N.O. contacts on the relay will alarm to the BAS. The other N.O. relay contact, in series with the N.C. reset switch(or is it a relay from the BAS?), will parallel the freezestat. When the relay is energized, it will hold itself on until the reset button is pressed or the power drops out (and the freezestat has reset). Which part are you struggling with if not the wiring?
These guys are on in. You'll need a NC momentary push button like described. Trane did this circuit factory on the old T Series AHUs, and I believe they carried it into the PCC indoor/outdoor. The old 2S6 push button with the 2K9 relay. Another example could be a motor starter with start and stop buttons instead of an HOA. The stop is usually a NC momentary that would be the same as your "reset" button in your circuit.
Here's a rough drawing of the circuit. The LLT relay would also have a set of contacts in the VFD/starter safety circuit, as well as a contact to the BAS for status.
I haven’t come across the following application before, where integral face and bypass steam heating coils shall have auto-reset freezestats and latching relays. I need to provide latching relays with LED indicator of relay status for each freezestat (10 total) so that the relay will remain latched after the freezestat resets. A momentary reset switch will be provided for remote contact reset from the DDC system for resetting the freezestat alarm. The relays shall be unlatched after a power fail.
I was planning on using DPDT relays for the latching circuit and understand how to create the latch using the first set of N.O. contacts on the relay wired in parallel with the N.O. momentary switch to keep the relay coil energized. However I’m struggling with how to achieve the above mentioned circuit in the real world and am hoping someone has experience doing this and provide some insight.
I typically have multiple freezestats (SPDT) in series with each other and a 24VAC relay coil (with one set of contacts as an input to the DDC controller) so that when any one of the freezestats trips it creates an alarm through the BAS system. The freezestat acts independently of the DDC system via a hardwire interlock and shuts down the system.
This might be old news but ACI makes a 50’ freezestats. Pain in the ass to install but less freezestats overall.
Here's a rough drawing of the circuit. The LLT relay would also have a set of contacts in the VFD/starter safety circuit, as well as a contact to the BAS for status.
Alternatively you could put the reset button in series with the LLT relay holding contacts. This way the reset button would do nothing as long as the freezestat is still tripped vs. momentarily resetting the relay and re-energizing it when the button is released.
Alternatively you could put the reset button in series with the LLT relay holding contacts. This way the reset button would do nothing as long as the freezestat is still tripped vs. momentarily resetting the relay and re-energizing it when the button is released.
That would really be a matter of preference.
For sure. More than one way to skin the cat. Although the downside to the LLT being N/O is if someone happens to unplug it or cut the wire it will never trip. Just happened to a customer and they lost 2 CHW coils. Old Trane T-series AHUs with this circuit. Someone either couldn't get the units running or pulled the plug while doing filter maintenance without knowing.
For sure. More than one way to skin the cat. Although the downside to the LLT being N/O is if someone happens to unplug it or cut the wire it will never trip. Just happened to a customer and they lost 2 CHW coils. Old Trane T-series AHUs with this circuit. Someone either couldn't get the units running or pulled the plug while doing filter maintenance without knowing.
Good point. I never though about it, but we always use N/C for our safeties, probably for that reason. If you wanted to make this a latching circuit you would have to have NC freezestat > NO reset button in parallel with NO holding contact of relay > relay coil.
Alternatively you could put the reset button in series with the LLT relay holding contacts. This way the reset button would do nothing as long as the freezestat is still tripped vs. momentarily resetting the relay and re-energizing it when the button is released.
Would do the same. Can't reset unless fstat is cleared.
ReplyOriginally Posted by Sonnen86
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