Definite Purpose?

[wiry]

Greetings brothers and sisters-- My first post here though I've been using the forum as a reference for quite awhile.

My problem today has to do with lighting control. I've got a string of 400W HID fixtures on a 208V circuit that I want to switch with a T101 time clock and a photo control. Total circuit amps: 16.1A. So I plan to incorporate a contactor to switch the circuit.

The confusion arises over what kind of contactor. A 2-pole NEMA type lighting contactor such as the Square D 8903LO20 rated for 30A is almost 6 times the cost of a Definite Purpose 8910DPA42 which is both resistive and motor rated.

What I need is a plan english (that is, for a journeyman field electrician) explanation of just what a "Definite Purpose" contactor is good for. How exactly does their design differ from other types? I've read the catalog blurbs ("...ideal for heating, air conditioning, refrigeration, data processing and food service equipment"), and the the discussions here, ("DP contactors are terrible choices for one-off applications because the ratings on the labels are even more application specific than IEC ratings are, so it's really easy to misapply them and end up with damaged equipment or a fire.") But I'm still confused. Why can I use them for a motor but not a ballast? (Or do I have that wrong?)

Frankly I don't see the problem with a DP as long as I amply over rate the device. Over the years I've used them for all kinds of gizmos without any trouble. Am I just a lucky guy? Should I increase my liability insurance?

On the other side of the building I've got another 16A HID circuit switched by an Intermatic T104 (no photo control). Recommended for outdoor lighting or anything else you can think of it's been running fine for years, and considering how inexpensive they are I can't believe there's anything special about it's contacts. (Which I see are rated 40A tungsten or inductive.)

For that matter I don't see where the lighting contactor is rated for Metal Halide HIDs. It just says fluorescent and tungsten. Aside from the fact that you can order multiple poles what makes them any better than a DP?

And while we're at it what about the SCCR? An important consideration, I've been told. But how is it calculated?

Any illumination you guys can throw on this is greatly appreciated.:smile:

 

[jim dungar]

You can pretty much use any contactor to control any load for which it is rated. By looking at the "base" rating of device you can get a clue as to what it was primarily designed to do. Different loads have different MAKE, BREAK, and CONTINUOUS ratings that is why there are so many different types of control devices.

A motor contactor is designed to start and stop the locked rotor current of a motor. A NEMA device is so drastically overbuilt that it is hard to misapply it. NEMA devices are ususally rated in operations per minute.

A lighting contactor is designed to handle the very high inrush currents of tungsten and ballast lighting but it does not need to break much more than normal full load. Most lighting contactors are designed with replaceable contacts and include a version with a mechanical latch.

A definite purpose contactor is primarly designed to handle resistive heating loads with low cycle rates like those found in heating systems and small motor loads like those in well pumps.

 

[LarryFine]

To respond from the other side of the fence ( ), I have used simple 2- and 3-pole relays to bypass burned contacts on multi-pole lighting contactors, and simply paralleled the coils.

By the way, you can improve the current-interrupting performance of relays by placing contacts in series (yes, series, not parallel). This effectively increases the contact-opening speed.

 

[augie47]

DP

DP

Wiry, I no longer have catalogs at my fingetips, but at one time, DP contactors were "recognized" ( UR ) and not "listed". In the real world this would be more important to inspectors and legal folks (in the event of a failure). UR equipment is recognized as a component in a listed assembly, but not necessarily as a "stand alone" product.
I've seen a number in use, and, from my experience they will not hold up nearly as well over the long-haul as a lighting contactor.
SCCR is an important aspect of such applications and I will try and research the forum for info as it has been discussed a length. Ratings of lighting contactors, in general, are low (some at 5000 aic) and I;m not sure the ratings on the DP are easily obatined. Again, important in the event of a failure, fire or injury.


edited:
Here's a thread that discussed in length: http://forums.mikeholt.com/showthread.php?t=86837&highlight=lighting+contactors

 

[Jraef]

LOL, the age old DP debate.

NEMA = "Find it, Fix it and Forget it!"
Overbuilt for the US auto industry to take the worst possible thing a 'Lectrician could throw at it without having to talk to the overpaid engineering staff about the application. Probably more than most people need, but cheaper in the long run if down time costs $1000 / minute and it takes 10 minutes to get an Engineer on the phone.

IEC = "You can be fine if you've got the time."
Born in Europe where everything has to be "rationalized" by a degreed Engineer. Needs to be engineered for mechanical and electrical life, voltage, current, duty cycle and operating conditions, but there are very specific charts and formulas made available in catalogs for whomever needs to do it, all based on defined IEC "utilization categories" such as AC-1 through AC-6a. Came in full force to North America in the 1980's when we started getting our asses handed to us on productivity of repetitively built machine controls that could be engineered once and built many times. Still not looked favorably on by most end users, but accepted because their buyers go for the lowest price and/or smallest package. By the way, HID ballasted lamps are under AC-5a if you want to get the proper ratings.

DP = Don't Purchase in lots of under 1000 pieces."
Intended for specific use on specific equipment for specific loads running for a specific amount of time (i.e. just long enough to last out the warranty, no more, no less) without specifically costing a penny more per hundred lot than is absolutely necessary. Every aspect needs to be looked at individually, i.e. making capacity, breaking capacity, duty cycle, environmental contamination, ambient temperature, vibration, voltage fluctuations etc. etc. etc. There are no charts to help you, the manufacturers just make their information available and the onus is on you to take all the necessary issues into consideration. Then when it's all said and done, it is only UL Recognized (UR) as augie47 said. That means that IF you are a UL panel shop, you may use them only IF you add them to your procedure and have the entire assembly, including the SCPD upstream and any other devices downstream, evaluated and engineered. The cost to the panel shop for that UL evaluation is reportedly over $2000.00 per line item now (I gave up my UL shop 13 years ago and it was only $200 then). The only reason why you can buy DP contactors individually is because they need to be available for specific replacement purposes, and even then it has to be an exact replacement or one recognized by the equipment manufacturer as a legal substitute. ANY OTHER USE OF A DP CONTACTOR IS TECHNICALLY A VIOLATION!. More and more inspectors are getting wise to this issue now that SCCR ratings are being enforceable in the new code.

And yes, you are NOT going to be able to get an SCCR rating of any kind on a stand-alone DP contactor, not even the "courtesy" 5kA for untested devices, because IT IS NOT UL LISTED IN THE FIRST PLACE! To get an SCCR rating you need to go with IEC or NEMA designed products and ask the manufacturers for a combination listing. That will usually be with fusing or THEIR OWN BRAND of circuit breaker.

 

[Bob NH]

I have used DP contactors in small custom systems that will have less than 50,000 operations in 10 years, and the contactors are rated at 500,000 operations. They are typically used at not more than half of rated horsepower and I have never had a failure in more than 10 years of service.

It's a waste of customer's money to spend 6 times as much for a contactor that the customer doesn't need and for which he can't possibly recover the value.

 

[Jraef]

I have used DP contactors in small custom systems that will have less than 50,000 operations in 10 years, and the contactors are rated at 500,000 operations. They are typically used at not more than half of rated horsepower and I have never had a failure in more than 10 years of service.

It's a waste of customer's money to spend 6 times as much for a contactor that the customer doesn't need and for which he can't possibly recover the value.

So you have added them to your UL procedure and submitted the entire assembly for system evaluation then? If so, good for you. If not, you are in trouble if your panels ever get inspected by an AHJ who knows what to look for. Doesn't mean it's bad, just not code compliant.

A bigger "waste" of a customer's money is for him to install a piece of equipment, have it inspected and get red tagged, have to remove it, send it back, have it rebuilt with proper components and reinstalled. When you start adding up the extra downtime the bean counters will get really excited.

Another wasteful scenario is that (God forbid) a fire starts, even for some unrelated innocuous reason, in that panel. The Insurance adjuster / fire investigator sees the source as your panel, sees that it is not UL listed, voids the insurance coverage. Don't forget that even if you are in a state where UL listing is not yet required (i.e. still using the 2002 NEC), UL stands for Underwriter's Laboratories and is directly associated with the INSURANCE industry, not the government. Most industrial indemnity policies will include wording to the effect of voiding coverage for incidents caused by unevaluated systems, i.e. those without UL listing or some other NRTL.

 

[wiry]

You guys are awesome. This forum is an outstanding resource! Thank you so much for helping me to better understand what all is involved here. I can see that I need to do some more studying on this subject especially on the question of SCCR's. And I'll look around and see if I can find a good deal on a used/reconditioned lighting contactor.

Meanwhile this is something where I could use a little more explanation:

you can improve the current-interrupting performance of relays by placing contacts in series (yes, series, not parallel). This effectively increases the contact-opening speed.

 

[steve66]

DP contactors don't have replaceable parts, and you usually don't get as many options like adding more contacts, etc. (Or at least you don't get many options you can add in the field.)

 

[steveng]

what is sccr's?

 

[steveng]

My problem today has to do with lighting control. I've got a string of 400W HID fixtures on a 208V circuit that I want to switch with a T101 time clock and a photo control. Total circuit amps: 16.1A. So I plan to incorporate a contactor to switch the circuit.



why dont you use the time clock to switch the 16 amps load?
the contacts on the timers are rated for more than 16 amps.

 

[mdshunk]

DP contactors don't have replaceable parts,...

So the contact kit I put in at 2:00 today in a 30 amp, 3-pole square D contactor didn't really exist?

 

[wiry]

Steveng
1st Question: Short Circuit Current Rating. It's all new to me too. (But then I'm still working out of a 2002 code book!) Here's a URL for a PDF that'll give you some skinny.
http://www.google.com/url?sa=t&ct=r...yK6glW9jt1tfdq6AA&sig2=aJvuEJ0-t1z_xQ2a2NIvhw

2n Q: The photo-control which would be in series with the time clock is single-pole only. So unless I contented myself with switching only one leg... But maybe somebody out there makes a two-pole photo that I'm not aware of?

 

[hardworkingstiff]

Meanwhile this is something where I could use a little more explanation:

Yea, me too!

Larry?

 

[mdshunk]

2n Q: The photo-control which would be in series with the time clock is single-pole only. So unless I contented myself with switching only one leg... But maybe somebody out there makes a two-pole photo that I'm not aware of?

Sure, there's two pole time clocks. Even bigger, if you want them, BUT... control circuits commonly switch just one leg. It's permissible as long as your contactor is switching all the legs of the lighting circuits. Don't sweat it. I personally think there's value in using something "normal" like the T-101, since the future repairs will go pretty quickly, since that's a dirt-common time clock.

 

[steveng]

My problem today has to do with lighting control. I've got a string of 400W HID fixtures on a 208V circuit that I want to switch with a T101 time clock and a photo control. Total circuit amps: 16.1A. So I plan to incorporate a contactor to switch the circuit.


is this lighting circuit for night use only?

if so, why do you need the photo cell in series with a contactor and time clock?

 

[wiry]

Yes, night only. Outside. Commercial. On at dusk. Off at closing. 120V control circuit. 208V Lighting circuit. T101 clicks on at oh say 4:30pm. 365 days a year. Photo clicks on when it gets dark which could be 5:00pm on a rainy winter evening or 9:00pm in the summer. Control circuit activates contactor. Lights come on. It works.

 

[steveng]

why not use the photo cell to energize the contactor only?

is the time clock necessary in this application?

using this method you could switch both conductors at the contactor, without an additional device like a time clock.

 

[eric stromberg]

why not use the photo cell to energize the contactor only?

The statement was "on at dusk, off at closing." So, the photo cell is used to turn the lights on, the time clock is used to shut the lights off. If he used a photo cell only, the lights would stay on all night.

 

[LarryFine]

Meanwhile this is something where I could use a little more explanation:

you can improve the current-interrupting performance of relays by placing contacts in series (yes, series, not parallel). This effectively increases the contact-opening speed.

Lisa: Would you like me to explain?
Vinny: I would love to hear this!
Judge Haller: So would I.

When a set of contacts open while conducting current, a good bit of arcing occurs; how much depends on the current, voltage, and type of load. The faster contacts under load are opened, the faster the arc is extinguished. Plus, it takes a higher voltage to initiate an arc than maintain it.

Anyway, when you place two sets of contacts in series, they effectively open at twice the speed, the arc voltage is shared between the two sets of contacts, the arc is extinguished faster, much less contact pitting occurs, because the arc duration is less, and the localized heating is lessened.

When minimal overall contact resistance is paramount, contacts can be placed in parallel, which we all know reduced the resistance, and voltage drop, proportionately to the number of pathways. However, to open a load current, placing contacts in series increases contact life and reliability.