Surge Protection for PLC

I have a IDEC PLC that is controlling 8 pnuematic solenoid valves, monitoring 2 4-20 ultrsonic tank levels, and displaying 3 digital flow meters. All of the peripheral equipment is exposed outside. The PLC and Touchscreen is mounted inside a control room.

My question is how and what surge protection should I use. My thoughts are to use a surge protector for teh incoming 120v power for the plc and power supply. I was also going to add a surge protector for the peripheral devices to protect from incoming surges...does this seem extreme or necessary?

my selected devices so far are:

Phoenix Contact: 2856812 120v surge, 2838173 2 point 24v surge protection, 2804623 2 pt analog protection

My question is how and what surge protection should I use.

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There is no one answer that fits all installations. You have to weigh the cost of additional surge protection against the probability of equipment loss and the ensuing costs. For many situations, it makes more sense to suffer the occasional loss. For others, any downtime costs many $millions.

Understand the concept

Surge protection is used wher there is chances of high current to mix up with our nominal PLC or Field instrument cabling.
So you have to survey the plant and decide according to it where is chances and for which instrument or field wiring are at risk. See cable laying. etc.

as Lancie, usually for small plc, it cost more to install isolator for analog signal than what would cost a replacement card so burning costly isolator or unexpensive module is easy to choose from but in case of a central plc you must think about it.

For discrete output, surge arrestor and reverse diodes modules cost fraction of 1 buck so it shouldn't be an option...

usually a good ground and surge arrester on exposed equipement/building is better because it will discharge the problem at is origin point...

lefeverj said:

My question is how and what surge protection should I use. My thoughts are to use a surge protector for teh incoming 120v power for the plc and power supply. I was also going to add a surge protector for the peripheral devices to protect from incoming surges...does this seem extreme or necessary?

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First, eliminate the idea that any protector does protection. Did you really think a protector would somehow block or absorb a destructive surges? Anything that might do that must already be inside electronics.

Second, a destructive surge does damage if that PLC is the best connection to earth. If a surge can blow through a PLC, then it easily blows through 'blocking' or 'energy absorbing' protectors. But if connected low impedance to single point ground, then nobody even knows a surge existed. That energy dissipates harmlessly in earth.

In most facilities, single point ground means wires inside every incoming cable connect low impedance (ie 'less than 10 foot') to the single point ground. All wires enter at the common (service entrance) location. All connect low impedance to one ground. Make that connection either directly or via a protector.

The term low impedance is critical. If a ground wire is inside metallic conduit, has splices, sharp bends, or bundled with other non-grounding wires, then protection is compromised. Also says why a wall receptacle safety ground is not earth ground.

That connection is either a direct wire connection (ie coax cable). Or via a protector (ie ethernet, RS-485, etc). Best protection is as close to earth ground as possible. And as much distance as possible between the protector and PLC.

Destructive surges can be hundreds of thousands of joules. Protection means you know where that energy dissipates. Again, no protector does protection. A protector only makes the connection. Protection is defined by low impedance to and quality of earth ground.

If this is a large factory, then more and careful considerations must be included. Some better facilities make the entire earth beneath a building into one big single point ground. Others have virtually no consideration because concepts are unknown. Such as impedance mistakenly assumed to be resistance.

If a surge need not find earth destructively via the PLC, then superior protection already in the PLC is more than sufficient.

Same rules apply to sensors in another building. Each building must have its own single point earth ground. Any wire that enters (or leaves) either building must make that low impedance connection to earth.

If these concepts are ignored, then a lightning strike anywhere to one building can become a direct strike to connected electronics in that other building. We are talking about transients that occur maybe once every seven years. A number that can vary significantly even with geology or how a building is constructed.

Protection is never defined by a protector. Protection is defined by what a protector connects to. A protector is only as effective as its earth ground.

westom said:

First, eliminate the idea that any protector does protection. Did you really think a protector would somehow block or absorb a destructive surges? Anything that might do that must already be inside electronics.

Second, a destructive surge does damage if that PLC is the best connection to earth. If a surge can blow through a PLC, then it easily blows through 'blocking' or 'energy absorbing' protectors. But if connected low impedance to single point ground, then nobody even knows a surge existed. That energy dissipates harmlessly in earth.

In most facilities, single point ground means wires inside every incoming cable connect low impedance (ie 'less than 10 foot') to the single point ground. All wires enter at the common (service entrance) location. All connect low impedance to one ground. Make that connection either directly or via a protector.

The term low impedance is critical. If a ground wire is inside metallic conduit, has splices, sharp bends, or bundled with other non-grounding wires, then protection is compromised. Also says why a wall receptacle safety ground is not earth ground.

That connection is either a direct wire connection (ie coax cable). Or via a protector (ie ethernet, RS-485, etc). Best protection is as close to earth ground as possible. And as much distance as possible between the protector and PLC.

Destructive surges can be hundreds of thousands of joules. Protection means you know where that energy dissipates. Again, no protector does protection. A protector only makes the connection. Protection is defined by low impedance to and quality of earth ground.

If this is a large factory, then more and careful considerations must be included. Some better facilities make the entire earth beneath a building into one big single point ground. Others have virtually no consideration because concepts are unknown. Such as impedance mistakenly assumed to be resistance.

If a surge need not find earth destructively via the PLC, then superior protection already in the PLC is more than sufficient.

Same rules apply to sensors in another building. Each building must have its own single point earth ground. Any wire that enters (or leaves) either building must make that low impedance connection to earth.

If these concepts are ignored, then a lightning strike anywhere to one building can become a direct strike to connected electronics in that other building. We are talking about transients that occur maybe once every seven years. A number that can vary significantly even with geology or how a building is constructed.

Protection is never defined by a protector. Protection is defined by what a protector connects to. A protector is only as effective as its earth ground.

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Well said!

Well said!

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Except for this part:

But if connected low impedance to single point ground, then nobody even knows a surge existed. That energy dissipates harmlessly in earth.

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A single point ground DOES NOT guarantee that lightning energy will be dissipated harmlessly in the earth. We all hope that it will be, but in many cases it is not. A lightning strike has so much power that it often follows the ground wire into the earth, but as it passes, side strikes and induced surges do tremendous damage to random equipment in the path.

It reminds me of a visit to an Army explosives plant many years ago. There were 10 production lines, widely separated by the SOP required minimum distance to prevent an explosion at one plant from propagating to the others. These facilities had the overhead wire grid style of lightning protection widely used by ammunition and explosives plants. This grid is a net of #00 AWG copper wires strung 5 feet apart and suspended on poles 25 feet above the tallest part of the plant. Each pole had a large ground wire going to the buried grounding loop. As we drove by the large hole where Plant #4 was supposed to be, my escort said "we lost that one to lightning a couple years ago".

There are no guarantees.

Lancie1 said:

There are no guarantees.

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Your local telephone switching center (CO) suffers about 100 surges with each storm. How often is your town without phone service for four days while they replace their computer? Never? Telcos use earthing to make 100 surges irrelevant.

No 100% guarantee exists. Because human mistakes create a compromised earth ground system. When damage occurs, the human should learn about his mistakes. That investigation usually begins with the earthing.

Telcos use earthing to make 100 surges irrelevant.

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The ones that I have visited also used telecom surge arrestors across each incoming terminal block, that try to shunt a surge above a certain voltage level to the ground bus! Even then the lines often get burned. Our local company has a router that tries to find another path for each call. This worked well except after the recent tornadoes that destryoed too many phone lines.

Lancie1 said:

The ones that I have visited also used telecom surge arrestors across each incoming terminal block, that try to shunt a surge above a certain voltage level to the ground bus!

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Look closer. Those protectors are not between wires. How effective is that protector? If failures are happening as described, then the informed human is busy locating and correcting his mistake. Or, like others who foolishly think a protector does protection, he has assumed damage is acceptable. Protection without damage from direct lightning strikes is routine.

Failure means a human has made a serious mistake. Often in what most humans are completely ignorant about. Single point earth ground. Most only know about what they see. For example, the less informed argue about pointed verses blunt lightning rods. What makes the lightning rod effective? Neither. Earth ground - the item they did not see and therefore ignored - defines building protection.

Same applies to electronics protection. A protector is only as effective as its earth ground. What so many don't discuss because they do not see it. Knowledge only from observation is usually deficient.

How long is that wire to earth? More than 10 feet? Then a human may have compromised protection. This stuff was well understood over 100 years ago. And still so many never learned it. Protection is not defines by a protector. Protection is defined by the quality of and connection to single point earth ground. Electricity did not change in 100 years.

Unfortunately you are making conclusions only from observation. Not from doing this stuff.

Look closer. Those protectors are not between wires.

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I have removed them, held them in my hands, replaced them, ordered more for new terminals. They are still the same - one surge arrestor for each Incoming terminal.

For example, the less informed argue about pointed verses blunt lightning rods.

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I think you are ASSUMING a lot more than you know. The shape of the rod has little effect. In fact, the most effective lighting protection system that I have ever seen did not use rods at all.

Lancie1 said:

I think you are ASSUMING a lot more than you know. The shape of the rod has little effect.

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Shape of a lightning rod does affect effectiveness. Hearsay promotes pointed rods. Science has demonstrated that blunt rods are superior. But the differences are trivial. Because that rod - like all protection systems - is mostly defined by the quality of and connection to earth.

No lightning rod or protector does protection - except where myths due to advertising have replaced over 100 years of well proven science. Protection means one states with certainty where hundreds of thousands of joules dissipate - harmlessly.

That is true for what protects the building - lightning rod. And also true for what protects interior appliances - earthing either directly (hardwired) or via a 'whole house' protector.

Either energy is absorbed harmlessly outside. Or it will go hunting destructively inside. Maybe destructively through a PLC. Well proven protection means a direct lightning strike. And everything (even a dishwasher, bathroom GFCI, and protector) remain functional.

That is true for what protects the building - lightning rod.

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Unless if it is a building that CANNOT be allowed to be hit by lightning, such as an ammunition or explosives plant. The newer ones do not use rods at all, but overhead wire grids, about 15 to 25 feet above the building. The really sensitive stuff, such as nitroglycerin and lead azide, are manufacutred in earth-sheltered bunkers.

Lancie1 said:

The newer ones do not use rods at all, but overhead wire grids, about 15 to 25 feet above the building.

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But again you are discussing what you see. Not what does protection. A superior solution was pioneered in muntions dumps. Ufer ground. Not that catenary, lightning rod, etc. Those protectors are only connecting devices to what does protection.

What defines the quality of a protector? Its protection - the earthing. What made protectors in munitons dumps even better? Ufer grounds.

"A protector is only as effective as its earth ground."

But again you are discussing what you see.

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No you don't know me and are making assumptions about which you know nothing. I am discussing what I have done and what I know that works.

What made protectors in munitons dumps even better? Ufer grounds.

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That is correct. I have been designing and using the Ufer grounding method for over 40 years, in all types of installations, from the sandy soil at an ammunition decon plant in the desert at Hawthorne, Nevada, the rocky clay at the Rocky Mountain Arsenal at Denver Colorado, and the sandy silt along the Missippii River in New Madrid, Missouri. The Ufer method works equally well in all those situations. The site safety department at Hawthorne could not believe the 2.6 Ohms measured from one of my site Ufer ground systems to a test rod, even though Herbert Ufer was working for the Army when he developed his method.