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FreezerGeezer
06-10-2009, 03:50 PM
Came across a job that's been driving me nuts. The unit is running fine since I repaired it, however the fault light on the site's display panel lights every time the unit runs.
No wiring diagrams of course, and the units have been retrofitted from heat pump to cooling only.
On being called out recently, I found the unit had no refrigerant, but also the LP switch was faulty.

Now, it was also wired such that, having traced out the wiring (which is a huge mess, and in a tight panel too!) it appeared that the switch was wired so that there would be a signal to run whether the LP was made or not. So I corrected the wiring (as I saw it) when I fitted the new switch, putting it across the normally closed contacts (Penn switch, 1 & 3 are the NC, 2 & 4 are the LP & HP fault contacts respectively). This is the only way I have ever seen (as far as I can remember) or been taught to wire LP/HP switches - you want a fault signal, you add a core to your cable spec. The signal then goes to power a relay that feeds the compressor contactor signal.
On being recalled to this 'fault light problem' today, I have racked my brains, and finally found legible copy of the original wiring diagram in another plant room (WTF isn't it with the unit it belongs to, or at least in a related plant room!!!!????? :mad:) and I think the LP/H switch is not directly in the control circuit, but paralel to it.
From the original wiring diagram and a lot of lateral thought (remember that the unit has been significantly altered from HtPmp to Cooling only) I now think that the way the switch is meant to be connected in this system is so that with no fault, there is NO signal through the switch. What I thought was the compressor relay is most likely the fault relay. So that when either of the safety switches break, making the fault circuit, this relay pulls in, and the signal for the compressor should be on the normally closed connection so that on activation of the relay, the compressor loses power.

Sorry for the long winded explanation, I wanted to make sure I gave all the info I can recall.

Now, as I said, I have always been taught, and as far as I know, always seen the HP & LP swicthes in series with the compressor contactor circuit. If I'm right about this circuit, they are effectively in parallel. I believe this is not best practice as the relay is another thing that may fail (let's face it, most of us keep a few spare relays in the van). And in this case if it fails, it fails UNsafe, in that although the switches are trying to stop the compressor, it can keep running.

So, have I gone mad? Is it actually a common way of wiring the safety system? Given a choice, would you wire it so that the HP/LP is in series with the compressor control circuit and use the relay purely for fault signalling, or would you wire it so that with the LP/HP made, the relay is OFF, and the compressor circuit is made through the relay's NC connection?

I look forward to your opinions. :)

acnerd
06-10-2009, 04:41 PM
Not that I can comment on the Australian way ;) .....

but here in the UK on the cold rooms I used to install and older fridge systems with 'proper' LP/HP switches they would indeed be in series with the control circuit. In effect breaking the 240V (or whatever voltage) signal to the solenoid coil on the compressor contactor. The signal connections would then go to an indicator lamp of the same voltage to show the fault.

You've lost me on the parallel circuit .....
Was the old switch auto-reset? Perhaps with the heat-pump and defrost periods it could cause the switches to activate. Just thinking out loud. :)

Grizzly
06-10-2009, 10:23 PM
Hi Freezer Geezer.
Basically acnerd is correct.
The run signal (live feed) should run through the safeties!
L.P., H.P., and if relevant Oil Diff for example.
The route it can then take can be complex.
Check out the attached pdf. page.
From a Danfoss manual.
This one shows 2 main safeties but you could Have High temp etc as well.

Check out the various room stats but there is always the safety circuit "IN SERIES".
Hope this helps Grizzly

Sorry I cannot upload the file as it is just above 100kb.
I will have to get back to you on that one.

FreezerGeezer
06-10-2009, 10:45 PM
Thanks a lot guys, I'm glad it's not me going mad! :D
I couldn't believe it when I worked out what was going on. Should be back there in the next few days, will report back.
Grizzly, the attachment isn't showing for me?

Now, how do I tell the client thier system is wrong without dropping the dept. in it? ;)

nike123
07-10-2009, 09:13 AM
From the original wiring diagram and a lot of lateral thought (remember that the unit has been significantly altered from HtPmp to Cooling only) I now think that the way the switch is meant to be connected in this system is so that with no fault, there is NO signal through the switch. What I thought was the compressor relay is most likely the fault relay. So that when either of the safety switches break, making the fault circuit, this relay pulls in, and the signal for the compressor should be on the normally closed connection so that on activation of the relay, the compressor loses power.


This is certainly not the case, since if it is wired as you described (compressor fed thru NC contacts of contactor), than compressor will be energized at moment when you supply power to unit.

Could you make picture of that schematic diagram and post here?

It is probably wired together with memory relay if HP/LP is automatic reset type.

FreezerGeezer
07-10-2009, 11:35 AM
I spent a number of hours tracing it all through today & drawing a wiring diagram of the current setup. Once I've got that legible / sensible ;) , I'll post it.

How the HP/LP (hensforth PSS) & fault relay is wired follows:

The 24V signal to the compressor contactor goes through a NC contact on the fault relay (referred to as a Lockout Relay on the original wiring diagram). The fault lamp circuit goes through a NO contact.
The PSS is fed from the live compressor connection on the relay, through the PSS fault connections (2 & 4) and back via the common live connection (1) to the A2 of the fault relay coil (by the way, can anyone tell me why people insist on wiring everything in reverse?).
The common return connection (3) is not used.

FreezerGeezer
07-10-2009, 11:39 AM
Thus, when there is a PSS fault, the feed circuit for the fault relay coil is made, which opens the compressor control circuit and closes the fault lamp circuit.

The reason I believe this is very wrong is that in my experience relays tend to be a bit fragile. Obviously in this case should the coil fail or the NC circuit become welded, the compressor can continue to run regardless of what the PSS is doing!

FreezerGeezer
07-10-2009, 11:41 AM
Nike, the PSS is manual reset, and the cooling signal comes from a thermostat. Hopefully all will become clear once I have the drawings up.


EDIT: Sorry for the multiple posts, for some reason when I try to post the above as one message, I keep getting an internal server error.

nike123
07-10-2009, 06:00 PM
Is this thread (http://www.refrigeration-engineer.com/forums/showthread.php?t=20909) helpful?

nike123
07-10-2009, 06:23 PM
Nike, the PSS is manual reset, and the cooling signal comes from a thermostat. Hopefully all will become clear once I have the drawings up.


If both HP and LP pressure-stats are manually reset, than I dont see purpose of memory relay.

nike123
07-10-2009, 06:29 PM
This is certainly not the case, since if it is wired as you described (compressor fed thru NC contacts of contactor), than compressor will be energized at moment when you supply power to unit.



Sorry, I wrongly understood that part of your description.

Grizzly
07-10-2009, 07:05 PM
Hi Freezer geezer.
It all makes absolute sence to me.
Particularly as the control circuit is 24v.
( Not intended to teach you to suck eggs )
A relay is a low load contactor after all.
Used more commonly where Modern controllers are used.
So the relay brings in a larger contactor, capable of switching the loads obtained when starting the comp!

With regard to A1 or A2 being live or Nuetral, it matters not on a Coil!
It usually ends up being decided by whoever wired the panel in the first place.

Just a thought if you wire the temp stat after the relay and before the comp contactor.
You have a fail safe on both!
Cheers Grizzly

nike123
07-10-2009, 08:09 PM
Thus, when there is a PSS fault, the feed circuit for the fault relay coil is made, which opens the compressor control circuit and closes the fault lamp circuit.

The reason I believe this is very wrong is that in my experience relays tend to be a bit fragile. Obviously in this case should the coil fail or the NC circuit become welded, the compressor can continue to run regardless of what the PSS is doing!


We could also say that, if pressure stat is faulty, than compressor will continue to work no matter what pressure is. But then compressor overload relay or termal protection comes to function and protect compressor from damage.
That is only one level of protection of compressor as vital part of system and it should be always more than one protection for such important part.

FreezerGeezer
08-10-2009, 12:20 PM
Is this thread (http://www.refrigeration-engineer.com/forums/showthread.php?t=20909) helpful?


We could also say that, if pressure stat is faulty, than compressor will continue to work no matter what pressure is. But then compressor overload relay or termal protection comes to function and protect compressor from damage.
That is only one level of protection of compressor as vital part of system and it should be always more than one protection for such important part.

Yes it was, thanks Nike.

And I agree with your second statement.

Here's the neatened wiring diagram. There's some wiring involving the PFR that I haven't put in. I don't think there's any more wiring attached to the SCT, although it rather looks like there should be if I've understood the circuit operation correctly.
http://i136.photobucket.com/albums/q187/ratel10mm/BrownsPlainsAC3.jpg

Grizzly, I understand your comment and yes of course it makes sense. This is why I originally thought the relay was part of the cooling signal circuit instead of being the fault relay. You'll see on the wiring diagram that the 24V cooling signal is switched on the fault relay (HPFR) for isolating the compressor in the event of a pressure fault - not, as I first thought, making the cooling call!

Anyway, my concern with the wiring is still that whoever designed it has introduced a fail UNsafe mode in the relay, when it should all be fail safe. While as far as I know this hasn't been a problem, these units are over 22 years old & not in great condition. So I intend to alter the wiring so that the HP/LP NC circuit will be where the HPFR NC is now, and will probably leave the HPFR where it is.

Comments?

iceman007
13-10-2009, 10:40 AM
Thanks a lot guys, I'm glad it's not me going mad! :D
I couldn't believe it when I worked out what was going on. Should be back there in the next few days, will report back.
Grizzly, the attachment isn't showing for me?

Now, how do I tell the client thier system is wrong without dropping the dept. in it? ;)

Just blame Carrier

Ahh to think what trouble it was to get a licence to work to these standards ????????

desA
13-10-2009, 02:31 PM
I'd not be happy with the HP/LP being connected in parallel that way.

I wire them in series e.g. Ranco LP/HP - on the series control line.

It could be that the manual LP/HP was dropping the machine frequently & someone became tired of having to reset the switch & so decided to wire it in parallel.

A replacement with an automatic reset LP/HP wired in series, would do the trick. The LP/HP switch is a great way of catching a low refrigerant charge, amongst other things.