Why are some manufacturers low voltage transformers & circuits wired to true ground?
Some manufacturers like Lennox have a dedicated ground from common of the low voltage side of the step down transformer other manufacturers don’t. I’ve always understood that certain ignition control boards and other components need a good solid ground in order to work properly. I’ve just never taken the time to understand or figure out why some manufacturers have it and others don’t. I’d like to be able to proper explain this to my technicians in the correct way of understanding it & not just “my way of understanding it throughout the years I’ve spent in the trade.
The question arose on a trouble call that one of my younger, newer technicians was on & needed help with. It’s a fairly interesting low voltage short problem that has a good story behind it & hopefully a good & successful ending! I’ll probably follow up this post with the details but for now a good question for those of you that know is this?
The manufacturers that use this “grounding of Common terminal” (low voltage), are they using a designated “proprietary” low voltage transformer to make this happen? When a technician replaces a bad transformer on a system that has a designated “C” terminal on the low voltage side of the 24v transformer, can he or she get by with using just any over the counter transformer (as long as it is the correct VA)? Even if that transformer does not have a designated common terminal?
Thanks,
TD
Grounding the common side of the transformer is fairly common from what I have seen. A lot of units are grounded through the board mounting so there is not a wire doing the grounding.
You will see this on a lot of circuit boards not just IFC boards on furnaces. I believe it is for better protection for the control circuit. If common is not grounded then a wire or component could shot to ground and the fuse not pick it up so the control circuit stays live. This could cause control circuit malfunction. Then there is also keeping the common side of the control circuit potential to ground "0" which helps the IFC with Flame rectification. +
Bonding transformer 24V common to ground is helpful for troubleshooting because you have a reference everywhere, it’s helpful for short circuit conditions (ie blows fuse when metal is cutting into wire), and is normal electrically. By normal I mean consider that mains neutral is bonded to ground and that’s why you can touch it without getting shocked.
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Not bonding the neutral teaches you to be a better tech.
"Is this before or after you fired the parts cannon at it?" - senior tech
I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
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Originally Posted by emelio67
Better to have a true ground.
Define "true ground". Is that the safety neutral bonding circuit in a T-49-HC?
"Is this before or after you fired the parts cannon at it?" - senior tech
I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
Unless we change direction we are likely to end up where we are going.
"It's not new, it's better than new!" Maru.
You can pick a generic step down transformer and choose one of the two leads on the secondary side to be your common, either one it doesn’t matter unless it’s already identified in some way. Then ground that lead. That is now your common. Wire it into the circuit similar to your existing transformer. There are some exceptions to this but for most 24v transformers it will ring true.
Quickly, I must hurry, for there go my people and I am their leader!
Allowing grounding thru electric circuit components can cause inconsistent operations. On old two conductor circuits often had to ground controls to water piping, two solid state controls in series may each need separate grounds. Ungrounded control transformers can back feed and bypass safeties.
Worst case was a Trane roof top that the xformer ground was loose hanging behind a component, on a call for heat the gas turned on with no safeties or ignition.
McQuay chiller with Seimens controller, 24 vt was looking for ground thru controller and causing controllers to fail.
I'm glad to hear about the phasing - I working a multi GSHP system with odd control problems.
I found floating low voltage grounds between them and if I measure VAC between them I get 24V from ground to ground and 48VAC (actually 28 and 56VAC) between the (non grounded floating) "hots. I swapped Legs on the 240V heat pump to get it and its dedicated zone controller on the same 24V phase but still have out of phase on other units. So I think I need to swap the supply breaker for the "odd" controller to L1 from L2 or visa versa.
I got it running on heat anyway. It still has control (and inter-component voltage) issues so I will return.
Transformer grounding depends also on the number of outputs and transformer applications, including testing. For safety and equipment proper functioning, the secondary side should be grounded.
‘If the transformer has one single voltage output, it is likely to be either 240V or 120V to provide power for loads, or it could be 24V AC, which is common for many control devices.
In either case, one of the two wires should have a connection to ground. This wire becomes the neutral wire. If there is only a single output voltage, it does not matter which wire is neutral and grounded, although sometimes a particular one will be preferred in order to maintain phase alternation between devices.
It is here with the single output transformers in which sometimes, but very rarely, there will exist a secondary winding that is ungrounded. This is only installed in order to prevent any sort of electromagnetic noise from traveling through ground near the output.
This is usually only connected in this manner when testing and troubleshooting equipment using grounding bench test devices such as oscilloscopes.
If the secondary of the transformer were grounded, in addition to the device under test, a ground loop may occur and cause damage. These are special variations of isolation transformers.’ - quote Control automation.
what is meant by that comment about teaching you to be abetter tech? ALL neutrals are bonded or they are not neutrals. most important connection in the world is the neutral to ground bond.
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Originally Posted by johnnyb
what is meant by that comment about teaching you to be abetter tech? ALL neutrals are bonded or they are not neutrals. most important connection in the world is the neutral to ground bond.
If 24 volt C is not bonded to "ground" the techs that put one lead on the case and check for power at various places in the circuit will be more confused than usual and will either learn proper meter use or be forever confused.
At least that's what was going through my head when I posted it.
"Is this before or after you fired the parts cannon at it?" - senior tech
I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
Unless we change direction we are likely to end up where we are going.
"It's not new, it's better than new!" Maru.
The reason that some manufacturers dont bond the control transformer to ground is that they haven't figured out that they are supposed to . It is required by NEC except in special applications for facilities that cannot be grounded and use a "special " (because I can't recall the name of it) system to detect ground faults. Look in NEC.
Check google for : "two girls killed in corn field in Indiana " DUE TO LACK OF BONDING THE SERVICE NEUTRAL OR MAINTAINING GROUND THROUGHOUT THE EQUIPMENT.This is less important at 24 volts but still important. I asked this very question as a first year apprentice 45 years ago. The answer in short is "So that the overcurrent device will operate"
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Originally Posted by emelio67
