class II supply

[junkhound]

Own home - Low voltage lighting,, using GE RR relays. After 45 years the class II 24 Vac transformer finally failed, internally shorted.

45 years ago power electronics just about did not exist, now one can get a 85V-265Vac to 24 Vdc converter for $10 !
Can set current limit to less than 1/2 amp on the unit to comply with class II, etc.

Question is: Is the use of one of these NEC compliant? It is UL listed.
I tend to interpret NEC to my own advantage (or ignore it altogether in some cases), but wondering how others would interpret this use. Standard cord plug-in, 24V output on external screw terminals.

 

[gadfly56]

Own home - Low voltage lighting,, using GE RR relays. After 45 years the class II 24 Vac transformer finally failed, internally shorted.

45 years ago power electronics just about did not exist, now one can get a 85V-265Vac to 24 Vdc converter for $10 !
Can set current limit to less than 1/2 amp on the unit to comply with class II, etc.

Question is: Is the use of one of these NEC compliant? It is UL listed.
I tend to interpret NEC to my own advantage (or ignore it altogether in some cases), but wondering how others would interpret this use. Standard cord plug-in, 24V output on external screw terminals.

If I understand you correctly, it sounds like you can set the power supply to operate outside the Class 2 limits. In that case I'd say it's not compliant because someone could change your setting. If I misunderstand what you mean, apologies.

 

[petersonra]

It is not a class 2 power supply unless it says it is.

 

[gar]

170503-1139 EDT

junkhound:

I was under the impression that you operated more than one RR coil simultaneously. One RR coil has a resistance in the range of 50 ohms. Two ir more will be less than 25 ohms. Or approaching 1 A. I don't know the AC impedance of single coil.

On my GE transformers I never load them with more than 500 ohms. Thus, if a switch gets stuck neither the transformer or relay coil will be burned out.

Over a 50 year period I have had several switches get stuck closed, but no coils or transformers have failed.

I much prefer the reliability of a simple transformer to a solid state power supply.

.

 

[junkhound]

You have a good memory for old posts Gar!

Yes, I operate up to 52 coils in parallel, one shot capacitor discharge sets the RR relays. Have about a total of 250,000 uF of Al electrolytics distributed at various places along the dc (35Vdc) line, rectified from the transformer to about 35 Vdc. Also need it to be dc for the extensive simple diode logic used for circuit control.

At first suspected a capacitor or diode shorted which loaded the transformer, but nothing amiss except the transformer itself failed. Diode good, cap(s) good.

Have done 'failure analysis' on class II xfmrs in the past, failure is usually that the windings were not potted sufficiently, and a shorted turn resulted from years of vibration due to 60 Hz magnetostriction.

I'm assuming any AHJ inspector would say no, so will leave instructions for grandkids if any of them ever live here to throw a class II xfmr on the wall if they ever need AHJ inspection. Maybe make a sticker that says 'approved for ClassII use?

 

[gar]

170503-1615 EDT

junkhound:

You have about 5000 ufd per coil if all 52 relays are operated at once. I believe about 500 ufd is enough. T = 500 * 50 = 25 mS. At 5000 we have 1/4 second.

I don't know how long one typically closes a switch to an RR relay.

Have you had any switch failures actuating 52 relays at once?

Also how much does line voltage drop when 52 relays are turned on at once?

Depending upon how much discharge occurs you could have some large peak current loading on the GE transformer for recharge.

If you added additional resistance in the charging circuit, then you could reduce peak load current on the transformer. With time limiting on discharge time to possibly 16 mS that would further limit the peak energy requirement per actuation of one or more relays at one time.

.

 

[junkhound]

Only switch failures I've had are the copper leaf in GE switches becoming oxidized, bit of scotchbrite fixes that.

Very seldom fire off all 52 at once, 4 or 5 at once a few times a week. The switches on anything more than 10 at once are 20A momentary dc rated toggle switches, not the GE type rocker switches.

I figure the 24 Vdc supplies are electronically current limited, so no overcurrent concern.


Ya know, I've never bothered to hook a scope up to a relay in the house to watch the current waveform, maybe should do that one of these days

 

[gar]

170504-1637 EDT

junkhound:

It appears that you and I are the only ones with RRs in our homes. iwire seems to work with RRs in what I suppose are commercial applications.

I would think that many more electricians would get involved with RR relays because of their potential capabilities.

I am amazed that you have had no significant problem simultaneously switching 10 RRs at a time with GE switches. This is roughly 5 A with a wimpy switch.

At what current limiting level would you set a regulated DC supply? I would guess at 0.5 = I^2 * 50 or about 0.1 A as a high value. This would be about 5 V across a coil.

.

 

[gadfly56]

170504-1637 EDT

junkhound:

It appears that you and I are the only ones with RRs in our homes. iwire seems to work with RRs in what I suppose are commercial applications.

I would think that many more electricians would get involved with RR relays because of their potential capabilities.

I am amazed that you have had no significant problem simultaneously switching 10 RRs at a time with GE switches. This is roughly 5 A with a wimpy switch.

At what current limiting level would you set a regulated DC supply? I would guess at 0.5 = I^2 * 50 or about 0.1 A as a high value. This would be about 5 V across a coil.

.

The relays are good for up to 20 amps resistive. Or are you talking about the 2XX series switches themselves? They are listed for up to 3 amps.

 

[gar]

170504-1915 EDT

gadfly56:

Yes I am referring to the control switch, RS2xx or whatever its part number might be, that switches the low voltage to the RR relay coil.

In an RR specification I found there was mention of need for a 50 mS pulse. Using 50 ohms and 500 ufd I get a time constant of 25 mS, and I believe from memory this is sufficient to operate a relay. I need to run some controlled tests.

I think it would be very rare for me to call a relay a switch. I would probably call it a relay contact, or relay output.

In my opinion the price for GE RR relays is very high for what is there and what manufacturing costs should be. Even worse for the control switches. From a cost standpoint these components can not be justified. Only when function is considered can there be any justification.

I believe the relay output contacts are rated 30 A resistive, and 20 A tungsten.

.

 

[junkhound]

Back in '71 the RR3 were about $2 ea IIRC. Over $25 now, but have a number of spares for home use snagged from surplus store onesies and twosies over the years.

Think the GE switches were 50 cents each back then, probably still have the old Graybar receipt somewhere.
Any switches I've added in the last few decades have been snap action microswitches.

I ordered a couple of 1A max 24 Vdc supplies from ebay/China ($5.98, including shipping). Only 2 settings, 0.5 and 1A, so will go with the 0.5 A. The shipping rates for a 2# package from China are still a wonderment to me, less than a postage stamp here.

 

[gar]

170505-0737 EDT

junkhound:

My guess is that the supply you ordered will have a moderately low internal output impedance, and hold fairly close to 24 V until current limiting at 0.5 A. Thus, an external load of 50 ohms will probably have about 24 * 0.5 = 12 W dissipated in the load.

I would guess that this much power dissipation on a continuous basis would burn out the relay coil, a stuck closed control switch.

Suppose the power supply internal output impedance is raised to 500 ohms, a resistor, then maximum steady state coil voltage is about 2.5 V. Coil power dissipation is about 6.25/50 or 120 mW. Instead with a 200 ohm internal impedance we get about 0.5 W dissipation. Possibly tolerable. But still a very long recharge time from a fully discharged 250,000 ufd capacitor.

Instead of increasing source impedance it might be desirable to add a time controlled limit on how long current could be supplied. For example, this might be a relay NC contact in series with the 24 V supply that opened after 100 mS of on current over 0.1 A, with an automatic reset time of possibly 1 second.

.

 

[kwired]

Own home - Low voltage lighting,, using GE RR relays. After 45 years the class II 24 Vac transformer finally failed, internally shorted.

45 years ago power electronics just about did not exist, now one can get a 85V-265Vac to 24 Vdc converter for $10 !
Can set current limit to less than 1/2 amp on the unit to comply with class II, etc.

Question is: Is the use of one of these NEC compliant? It is UL listed.
I tend to interpret NEC to my own advantage (or ignore it altogether in some cases), but wondering how others would interpret this use. Standard cord plug-in, 24V output on external screw terminals.

45 years ago we didn't have art 411 for low voltage lighting systems.

Today 411.4 would require all parts of the system to be listed for use together as a system.

I have no idea how to comply with your 45 year old system as it probably isn't listed, no longer meets current listing requirements, or no longer has listed components of that system available.

 

[gar]

170508-2205 EDT

kwired:

How does 411.4 apply?

A 24 V relay coil is not a light.

.

 

[kwired]

170508-2205 EDT

kwired:

How does 411.4 apply?

A 24 V relay coil is not a light.

.

Wasn't thinking about controls, but OP is asking about replacing a transformer that failed, and power supplies for LV lighting does need to be listed as part of the system, so unless this is for control power only it probably still applies.

 

[gar]

170508-2405 EDT

kwired:

If junkhound used the GE transformer sold for a power source for RR relay coils, then it was a Class 2 power source. This had an unloaded AC output voltage of about 24 to 28 V, and when rectified and filtered the DC voltage is in the mid to high 30 V range.

At this moment my unloaded DC voltage is just under 39 V.

In junkhound's application I believe he has in excess of 70 watt-seconds of energy storage at the DC level. It will be well above 70 if he is running at my voltage.

My guess is that junkhound may control as much as 50,000 W with one control switch 50 diodes, and 50 RR relays. Needs a good size breaker box. Is this on a 15 kVA power company transformer? I doubt it. In years passed my transformer was a 25 kVA unit until it leaked. The replacement is 50 kVA and I don't exceed 2 kVA much of the time. Most of the time I am below 24 to 39 kWh per day. I have a gas range top, a gas clothes dryer, and gas heat.

.

 

[kwired]

Sorry, should have looked to see exactly what a RR relay was before presuming this is a low voltage lighting system.

I now see he only has a low voltage control system for his general lighting.

 

[junkhound]

Update:
So, after a week or so the light switches again fail to operate, and the LED indicator on the 0.5A 24 Vdc supply is dim.

Ohm reading on feed to relay circuits was 4 ohms.

Aha, dendrite in the alum electrolytic capacitor ?

Hooked up a 20A 24 Vdc supply temporarily, all works, take the 20A supply off, now system works with the 1/2 A supply.

Must have burned the dendrite open, time to replace the caps (> 30 years old) before another dendrite forms.


Am assuming a continuous 4 ohm load on the original class II xfmr apparently overheated it to failure. Dendrite likely made a complete path sometime during the night, as the lights worked when we went to bed and were dead in the morning.