Controlling a panel with a relay

[gar]

080501-1347 EST

George:

I assume by ice cube relay that you are talking about something like a Potter & Brumfield KUP series. The per contact rating for these relays is 10 A AC resistive with silver-cadmium-oxide contacts and 5 A AC with silver contacts.

I would doubt that these would be allowed as a branch circuit contactor. Surface paths are short and contact spacing is small.

If an inspector would approve a KUP controlling a branch circuit I would still consider it a poor choice.

I beat KUPs to death in a DC clutch control circuit. This is a 1 A load at 108 V DC. A very brutal application but it provides relatively fast drop-out time. The clutch has a lot of inductance and has a 500 to 1000 #-ft torque capability. We parallel the contacts and they are switched from a couple times to maybe 10 times per minute. This is in a machine that builds pinion preload on automotive axles. Life of the relays is 3 to 6 months. In this time a considerable conical mound builds on one contact and a mating cavity on the other from the DC.

Replacing these relays gives the electricians something to do associated with the machine. Otherwise we run for years with virtually no problems. In reality low failure rates are a problem because the electricians are lacking experience with the workings of the control.

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[George Stolz]

I assume by ice cube relay that you are talking about something like a Potter & Brumfield KUP series.

I have no idea what that is. Here's an ice cube relay:

The per contact rating for these relays is 10 A AC resistive with silver-cadmium-oxide contacts and 5 A AC with silver contacts.

The relay I used was rated 10A @ 120V, yes.

I would doubt that these would be allowed as a branch circuit contactor. Surface paths are short and contact spacing is small.

Why not? As long as it's rated for interrupting the load it's working...?

I beat KUPs to death in a DC clutch control circuit. This is a 1 A load at 108 V DC.

Kinda apples and oranges, no?

 

[gar]

080501-1955 EST USA

George:

Yes your photo would be comparable to a KUP.

What happens to this relay with a dead short on the output?

Yes it is somewhat like comparing apples and oranges, but it does imply good durability for low current applications. In most cases where this type of relay is used the maximum short circuit current is way below that of a residential service, or is near the load and is probably protected with something more likely to interrupt the circuit at a much lower short circuit current.

Look at the wires connecting to the contacts. These are going to fry easily. The wire looks like #22 or #20. The fusing currents for this size wire is in the range of 40 to 60 A.

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[George Stolz]

What happens to this relay with a dead short on the output?

Then the OCPD should clear the fault before the equipment is damaged, I would think.

In most cases where this type of relay is used the maximum short circuit current is way below that of a residential service, or is near the load and is probably protected with something more likely to interrupt the circuit at a much lower short circuit current.

Why do you say that? The available fault current in commercial settings are usually far higher than residential premises. They do not specify that their product must be protected with any special OCPD.

Look at the wires connecting to the contacts. These are going to fry easily. The wire looks like #22 or #20.

And this is typical for many appliances, where we have small conductors connected to 15- or 20- amp branch circuits; this is where NRTLs step in to allow us to sleep at night over these issues, IMO. This relay is listed to control loads of 10A at 120V, so I do not see the concern here.

The fusing currents for this size wire is in the range of 40 to 60 A.

I do not understand this statement.

 

[iwire]

First of all, I think it's a terrible idea, although nobody asked my opinion on that.

No, they did not and lets at least get to the answer before trying to re-engineer the job.

Customers have lots of things they want, if what they want is within my ability to provide and is code compliant I have no problem doing what they ask. :smile:

 

[iwire]

Awhile back, I installed a simple ice cube relay in the loadcenter in my house, to toggle the occupancy-sensor having master control of my lighting (reference this thread for more details).

George in general 'ice cubes' are not used to directly control loads. They are 'pilot' relays.

Generally they control other ice cubes, inputs to PLCs or other larger definite purpose contactors or motor starters.

Now that said, it's your own home and I can't say I would not do the same for myself. :smile: But for a paying customer they get something a bit more rugged then a ice cube directly switching a load.

 

[Jraef]

Then the OCPD should clear the fault before the equipment is damaged, I would think.

And this is typical for many appliances, where we have small conductors connected to 15- or 20- amp branch circuits; this is where NRTLs step in to allow us to sleep at night over these issues, IMO. This relay is listed to control loads of 10A at 120V, so I do not see the concern here.

In your earlier drawing, you show a 15A breaker protecting a relay rated for 10A. That is the problem here. Somewhere in the specs for that relay will be the maximum fuse size itcan be used with. If you had a 10A breaker or a 10A fuse in between, then probably no problem. But 15A runs a risk of long term over load causing the relay to overheat and catch fire before the OCPD opens up.

Appliances that use smaller wires / relays / components etc. are typically provided with smaller circuit breakers or fused inside.They can even get away with a "fuse wire" in fact, a sufficiently small wire that will open under too much load, but do it in a safe manner. NRTL listing is only about not starting a fire, not quality, suitability, ability to repair etc.

As many ice cube relays as I have seen fry and catch fire, I would never do what you have done there unless I had a smaller fuse.

 

[gar]

080502-1856 EST

George:

From my quote and your response:

Originally Posted by gar
In most cases where this type of relay is used the maximum short circuit current is way below that of a residential service, or is near the load and is probably protected with something more likely to interrupt the circuit at a much lower short circuit current.

Why do you say that? The available fault current in commercial settings are usually far higher than residential premises. They do not specify that their product must be protected with any special OCPD.

Primarily I was referring to industrial settings and what I said was way below. Typical control transformers might be 1 KVA. Nothing close to 10,000 A short circuit.

Here is the reason: Large plants are generally 480, and smaller ones 240. In virtually every machine there will be a control transformer that steps the high voltage down to 120 or in some cases a power supply from the control transformer to provide 24 V DC. In these machines the main disconnect from 480 is fused. I have never seen a circuit breaker at this point. Any high power equipment will be fed thru power relays and additional fuses. The control transformer will be fused at the input and additional fuses on the output for the various 120 circuits. Virtually all logic and control of high power relays is at 120 or lower. Even here it is not usual to use KUP type relays. In older days much more rugged relays were used for logic than the KUP type. Today logic is mostly done with programmable controllers. A KUP type relay is more likely used in electronic equipment.

Others have more or less answered your other comments to me.

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[George Stolz]

I got to thinking about that little wire in there, and thinking about various aspects of this, and came to the conclusion that it probably wouldn't be a good design to install in someone else's house.

But!

I also got to thinking, is it illegal?

First, I looked to regular snap switches to try to determine that.

404.14(A) Alternating Current General-Use Snap Switch. A form of general-use snap switch suitable only for use on ac circuits for controlling the following:
(1) Resistive and inductive loads, including electric-discharge lamps, not exceeding the ampere rating of the switch at the voltage involved
(2) Tungsten-filament lamp loads not exceeding the ampere rating of the switch at 120 volts
(3) Motor loads not exceeding 80 percent of the ampere rating of the switch at its rated voltage

These don't refer us to the amp rating of the OCPD when choosing the switch, but to the amp rating of the load.

But then, I started to remember this conversation with Jim Dungar and went looking for it.

110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.

110.10 Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.

So, this leaves me thinking about two modes of failure: overload and fault.

It appears to me we have no code requiring us to worry about overloading this relay, aside from picking the loads it controls (if we were to borrow the concept from regular switches). Right?

And fault currents would be about the same between a 15A and 10A circuit breaker, right?

In your earlier drawing, you show a 15A breaker protecting a relay rated for 10A.

The contacts are rated for 10A. Does the coil side of the relay (shown as being supplied by a 15A circuit) have a max OCPD specified?

Gar, I understand what you're saying now.

I appreciate everybody's help with this, I'm learning here. Keep it coming.

 

[iwire]

George I look at it this way.

Use an ice cube, likely code compliant. A short or ground fault happens on the load side of the ice cube, maybe just a lamp that faults when it blows.

The result is the ice cube expires and now needs replacement.

Use a properly rated definite purpose contactor like these

or a lighting contactor

and you get that same fault the contactor survives, you clear the fault, reset the breaker and all is good.

 

[480sparky]

After re-reading the OP, I have a few concerns here.

One, it's an apartment. You'll need management's OK to do anything. And something like this, they simply won't understand, so they'll flat-out say 'No." Even if they agree, I seriously doubt you'll live long enough to see a monetary payback on whatever is installed.

Second, we're not talking an ice cube relay. Not even a 30a-rated lighting relay. Chances are, it's at least a 50a panel, maybe 100. An ice cube relay clicks. A relay capable of controlling 50-100amps is going to annouce your arrival & departure to each and every person in the entire building when it opens/closes.

Third. Once you open the relay, where are you going to get power from to close it again?

 

[George Stolz]

Ken, I think the discussion progressed into shutting down the branch circuits within the panel, to keep the fridge running while they were out.

 

[George Stolz]

George I look at it this way.

Use an ice cube, likely code compliant.

Bob, I really want to remove all doubt. Is it code compliant?

I agree on the design aspect of it, but I'm curious from a minimum code perspective.

 

[iwire]

Bob, I really want to remove all doubt. Is it code compliant?

I don't see why it would not be, as has already been mentioned the NEC / UL etc are interested in preventing fires, not the longevity of the equipment.

That said I have seen factory made control panels use ice cubes to directly control motors and such. They certainly cost less and pack densely when you have limited space.

IMO it all boils down to $.

 

[gar]

080504 EST

In a different thread I determined the source impedance at my home for three different locations. The lowest was just a few feet from the main panel and was 0.087 ohms. This would produce a short circuit current of about 1380 A from 120 V. One at a greater distance was 0.6 ohms and its short circuit current would be about 200 A.

The maximum possible current thru a relay after the small wire burns out and an arc is generated would be quite different depending upon the relay location.

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