Surge protection general question

[deanfran]

So as I have mentioned here before I have been attempting to gain a better understanding of PLC's and automation and control systems. As part of this I have spent the last couple weeks digging in to surge protection for electronic equipment. As part of this I started looking at some of our equipment at work. One system caught my eye for apparent lack of any type of surge protection. There is an AB MicroLogix 1200 (1762-L40BWAR)whose 24 VAC relay outputs are tied to a bunch of intervening relays (IDEC RU4S-A24) that in turn switch various loads. From what I can see there is zero TVS across any of these 24 VAC relays. This piece of equipment has been running 24/7 for about 3 years now, and we have never had a problem. IDEC does offer relays with built in suppressors, but this particular model doesn't have them. This control panel and everything in it was part of a pre-built package system, that came this way from a company that builds these things all the time, so I assume that I am failing to understand something here. What am I missing?

 

[OkiePC]

The current rating of that relay coil is listed as 37.5mA with inrush of 60mA which is so far below the rating of the ML relay outputs, that adding surge suppression is probably unnecessary.

 

[deanfran]

Okay that makes sense, but isn't TVS suppression also about protecting the PLC outputs from CEMF when the relay coil magnetic field collapses when the coil is de-energized? I thought that was a bigger problem than starting current.

 

[icky812]

Q. How big of a spike does it produce when the relay de-energizes?

A. It depends on where it is at in the cycle with regard to voltage and current.

Surge protection should ALWAYS be used with coil driven devices regardless of whether they are AC or DC powered. CEMF spikes (the spike that occurs when a coil de-energizes) account for a good deal of damage to equipment annually.

Please do yourself and your equipment a favor and use surge suppression.



As to why it happens: How is electricity produced? By passing a wire through a magnetic field. When a coil energizes it produces a magnetic field that does the work by pulling in a contact or plunger or other physical device. When electricity is removed, where does the magnetic field go? It does not just "disappear". It collapses back across the coil in a direction in reverse to which is was created.

When that magnetic field collapses back across the coil when power is removed it creates voltage. Often this voltage is many times the strength of the voltage with which the relay was initially powered due to the speed with which the field collapses as well as the amount of magnetic flux that the field contains. This in turn creates a counter electromotive force (CEMF) to the original polarity of the coil. This CEMF spike of voltage must go somewhere to dissipate.

With semiconductor devices, it seeks out the easiest path to dissipate. Often this is across a transistor or other sensitive electronic component. As this happens if the device is a silicon based semiconductor the junctions start to break down. Eventually the device fails due to either an open or a short from the excessively high voltage that the device was never designed to handle. At that point the user gets to spend some money to replace the device. Be smart and protect your electronics.

 

[deanfran]

Thats the part I don't get. Even ML 1200 manual agrees.

Because of the potentially high current surges that occur when switching
inductive load devices, such as motor starters and solenoids, the use of some
type of surge suppression to protect and extend the operating life of the
controllers output contacts is required. Switching inductive loads without
surge suppression can significantly reduce the life expectancy of relay contacts.
By adding a suppression device directly across the coil of an inductive device,
you prolong the life of the output or relay contacts. You also reduce the effects
of voltage transients and electrical noise from radiating into adjacent systems

And then the manual goes into great detail about how and which type of (Allen Bradley of course)suppressors to use. As I say the company that built this panel builds hundreds a year, and must know what they are doing, so I am confused.

 

[OZEE]

Is it the right thing to do? Yes
Will it work fine for years without any problems without having TVSS? Yes

In 25 years in this industry, I've seen them used maybe twice... no more than three times. Yes, in a perfect world they would be used, but they simply aren't in real life.

The company that made the machine is an OEM. They are interested in getting a good product at a very competitive price. One thing that we OEM's do to reduce cost is to eliminate those types of items unless a customer specifically requests them.

 

[harryting]

I'm with OZEE here, I have never seen them used on small interposing relay while I did see those took out a AB 1500 when it was driving sizable solenoid valve.

 

[Mickey]

Q. How big of a spike does it produce when the relay de-energizes?

A. It depends on where it is at in the cycle with regard to voltage and current.

Surge protection should ALWAYS be used with coil driven devices regardless of whether they are AC or DC powered. CEMF spikes (the spike that occurs when a coil de-energizes) account for a good deal of damage to equipment annually.

Please do yourself and your equipment a favor and use surge suppression.

I agree with icky812, use them. Save yourself some grieve down the road. Cheap insurance.

 

[deanfran]

Okay so OEMs are always trying to cut costs. I get this. Everybody is always trying cut costs. A Littlefuse 5KP16A is $3.39 if you buy more than 10. There are 15 24 VAC relays in this system. A little less than $51 to protect all of them. It seems pretty cheap compared to replacing a CPU. Is the whole TVS thing overblown? As has been said, they are rarely used. Is this just a matter of CYA by electronics manufacturers?

 

[leitmotif]

I thihk
1. You do not need fuses, circuit breakers, in electrical or brakes on cars to make them work.
2. You need to consider the WHAT IF factor.

What is the cost and implications of a PLC losing a contact because you did not protect it from the collapsing field of a coil?
Airplane crashing maybe ya otta do it.
Light bulb in parking lot not working - maybe who cares?

It all comes down to cost versus reliability. You are the guy on the scene it is your call and decision.

Dan Bentler

 

[icky812]

It doesn't have to be a $3 part. If it is 24VDC a simple 1n1001 reverse biased diode is better than nothing. For AC circuits a cheap MOV (Metal Oxide Varistor) is better than nothing.

If you can afford it, go with transorb diodes. They give great protection and won't breakdown as fast as other surge suppression devices.

I don't know the number of times I have told a customer that his broken PLC output or blown up touchscreen was caused by no surge protection on a coil. What I do know is that if I had all of the money that was wasted by not wiring in a $0.10 part on a coil to catch the CEMF surge, I wouldn't be here now.

 

[stealthelf]

It doesn't have to be a $3 part. If it is 24VDC a simple 1n1001 reverse biased diode is better than nothing. For AC circuits a cheap MOV (Metal Oxide Varistor) is better than nothing.

If you can afford it, go with transorb diodes. They give great protection and won't breakdown as fast as other surge suppression devices.

I don't know the number of times I have told a customer that his broken PLC output or blown up touchscreen was caused by no surge protection on a coil. What I do know is that if I had all of the money that was wasted by not wiring in a $0.10 part on a coil to catch the CEMF surge, I wouldn't be here now.

icky812

This is true but many Rockwell I/O cards come with surge suppression built in. No ADC products come with built in surge suppression and I think this is confusing for many customers that buy various brands as some have built in and some don't.Most people fail to do their homework in that area IMO.

What about productivity 3000 I/O modules? Any suppression built in on those? IIRC from the original design the answer is no but that may have changed?

 

[icky812]

ADC doesn't build it in for the simple fact that if it fails, there is no way to change the component and nor is there a way to isolate it to test it prior to failure.

Even if it were built into a card, I would still install the suppression at the device. That is where it really needs to be installed.

 

[westom]

Is the whole TVS thing overblown? As has been said, they are rarely used. Is this just a matter of CYA by electronics manufacturers?

View those relay contact voltages with an oscilloscope. Don't make decisions based only upon speculation or rationalization. View (or confirm) what actually exists.

 

[deanfran]

Now I am more confused. I have attached an excerpt from an AB Application Data document titled

Industrial Automation Wiring and Grounding Guidelines

I have highlighted a couple of passages that seem to indicate that AC output modules only require surge suppression if the solid state output module switch is in series or parallel with a "hard contact" such as a push button or selector switch. Sorry about the poor document quality. I had to reduce the quality to get under the forum file size limit.