Lightning Strikes Effects on Grounded DC Supply

A

Thread Starter

aminsameipoor

recently there is a critical dilemma in our power plant. many time we have experienced malfunction in electronic devices during lightning that makes outage of units. in addition the -12vdc of power supply is grounded, and we guess that it might be from transferring lightning voltage strikes through the -12vdc and ground system.

so i would like to ask the solution for this situation.

thank a million.
 
Lightning is tough, but not rocket science. In the end, you must enable a 'lower impedance path to ground' than your circuit. So in your example, if lightning is passing through your 12vdc supply, then it is either ENTERING the system elsewhere and the surge exiting your supply to seek ground - or it is entering via the ground through your 12vdc supply, while seeking to piggyback across your signal/comm lines to a remote ground.

I can't give a solid answer, but some clues:
1) radio and fiber optics (without metal armor) are ideal lightning-proof ways to links equipment.

2) adding as much simple galvanic isolation in an many places helps reduce the enticement for lighting to move through your system - so make sure your AC/DC supplies are isolated, use isolated 232/485 repeaters or converters, etc.

3) go online and read the support docs/white papers at companies who sell lightning protection devices. But be ware that lightning protection is ALL or NOTHING. For example, just adding lightning protection to your 12vdc supply alone could make the problem much worse. You need to look at all places your system 'touches' or 'nears' ground potential, and make sure lightning has a safe way to pass without treating yoru equipment as expensive fuses!
 
I wonder you have installed DC surge arresters in supply rails. Analyze your circuit and install surge arresters as required.
 
"Grounded" means connected to 'a' ground. Not all grounds are the same - they all have impedance thus when current of a strike flows they can be a wildly different potentials.

There are many examples where connecting everything to 'a' ground caused failures. Proper grounding requires a pavilion. Often, you can cause problems by grounding a rail of a DC power supply - depends on the total system.

I once wrote an article about this in regards to transducer failure.

https://wiki.xtronics.com/index.php/Lightning_Failures_in_Transducers
 
B

Bob Peterson

Just a few comments.

Lightning is a tricky thing to deal with. It pretty much does as it pleases regardless of what you do to try and stop it, or redirect it.

For the most part, "grounding" does little if anything to prevent damage from lightning. In fact, depending on exactly what is done it can make it worse. Most of the lore surrounding ways to ground things to prevent damage from lightning are either just plain wrong.

No one can tell you what is going on with your system or how to correct it from the information you have provided.

Is this a plant subject to US standards?
 
K

Karl Schmidt

> Just a few comments.
> Lightning is a tricky thing to deal with. It pretty much
> does as it pleases regardless of what you do to try and stop
> it, or redirect it.

I need to disagree. Not to pick on you - I've heard other well educated engineers say similar things - but lightning follows the laws of physics - follows ohms law. I've fixed several problems due to transients. The problem is people don't get hit every day and when they do, they buy a crow-bar arrestor and might get lucky for a year, never figuring out what the real issue was.

>For the most part, "grounding" does little if anything to
>prevent damage from lightning. In fact, depending on exactly
>what is done it can make it worse. Most of the lore
>surrounding ways to ground things to prevent damage from
>lightning are either just plain wrong.

True in part - proper grounding can provide huge benefits. Improper/inappropriate grounding can cause huge problems. Most sites, most homes, most factories do not have proper ground pavilions. Every service entry point can bring in a separate ground - phone lines, power lines, gas lines, cable TV, water lines, conductive structures. Understanding the importance of bringing everything in at the same place, sharing a ground pavilion, isolation from other grounds etc. These details can provide huge increases in reliability.

There is a large amount of published research on controlling lightning - really maters when they are storing explosive materials. What bothers me is when well trained people get superstitious when dealing with lightning - the laws of physics are not suspended when it rains outside.

Water towers and water plants can end up with lots of problems - pipes in the ground can end up grounding the power grid in rural locations. I helped one company change their practices and they ended up being able to offer 5 year warranty including lightning damage.

You are quite right that there isn't enough information here for anyone to help - most of the time it takes a trip to do a physical inspection - a look at the wiring diagrams - and then checking to see how is was actually wired (did they daisy-chain the grounds or make home runs to a pavilion? There usually was a plan - but often how it got wired is different. )
 
B

Bob Peterson

I won't argue with most of what you said. However, the high rise time nature of actual lightning strikes makes it very hard to do anything that does much good when there is an actual lightning strike short of paying a bunch of money for a full blown engineered LPS, and even then it is a bit of a crap shoot.

The worst thing I see people doing is trying to have "isolated" grounds all over the place on the theory that somehow this will help them. You can't really get much in the way of protection from lightning by sinking a few extra ground rods through the building.

As for service entry points having separate grounds for different services (electric, cable, phone, etc.), that is specifically forbidden by the NEC, and is the reason why a system bonding bridge is now required to have a place to tie them all together. if people are not building their electrical systems to the minimum standards of the NEC, it is unlikely any patches are going to work.

As far isolation from other grounds, that is forbidden by the code, so even if you wanted to do it for some reason, it would not meet code.

I am not sure what you are calling a "ground pavillion". is that what the rest of us call a grounding electrode system? If so, you really don't have a lot of choices as to what gets connected to it. The code pretty much tells you what has to be connected to it. About the only thing substantial that is metal underground that isn't typically required to be connected to the GES is the incoming gas pipe (in fact the code prohibits one from connecting it to your GES). Generally there is a fitting at the meter that isolates the building from the incoming metal gas pipe (if it is metal - most these days are non-metallic).
 
W
You have not given us near enough information to really help you out other than broad generalities. Generally, solving this type of troubleshooting problem is a matter of looking for clues as to how the electrical energy entered the system (through the power system, through the ground system, through the air, conducted through wires from somewhere else, etc.) to cause the damage and then blocking or removing that path or strengthening or protecting the victim device. A good ground system is a bulwark against the effect of lightning and the effects of lightning do not come out of the ground to cause damage without there being other problems related to the ground system and/or design of the electrical system. Some of the information that would be helpful is:

1. What is connected to the +/-12 VDC supply?

2. What electronic devices malfunctioned and how? Were they damaged and if so how were they damaged?

3. Were there similar electronic devices on the -12 VDC supply did not malfunction and if so, how were they physically related to the malfunctioning devices in the power system and ground system?

4. Were there electronic devices not connected to the -12 VDC supply that were affected by the lightning?

5. What type of ground system do you have, i.e. a general description?

6 How was the -12 VDC supply connected to ground. Does it go to an isolated "clean" instrument ground? (a violation of the NEC in the US and for good reason)

7. Has this happen before? If not, what was different? If different, were there any changes in the electrical system? Difference in intensity or location of the storm?

8. What protection against lightning or power disturbances do you have now?

9. What type of electrical ground system do you have if not in the US or under NEC?

10. Was there visible evidence of arcing in the system?

11. Were all the devices on the same branch DC circuit?

12. How were the devices connected to the ground system (e.g. equipment ground, case ground, etc.)?

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.
 
>... However, the high
>rise time nature of actual lightning strikes makes it very
>hard to do anything that does much good when there is an
>actual lightning strike short of paying a bunch of money for
>a full blown engineered LPS, and even then it is a bit of a
>crap shoot.

There are places that produce and store explosives on a regular basis where these problems have been solved. The fast rise time - while a problem for crow-bar protectors, provides an opportunity to filter out transients via inductors.

The reason it seems a 'crap shoot' comes from not recognizing both common-mode and differential mode transients. Both problems need to be addressed. Putting crow-bar protectors here and there can sometimes move the problem, but without an over-all plan, not likely to solve the problem.

>The worst thing I see people doing is trying to have
>"isolated" grounds all over the place on the theory that
>somehow this will help them. You can't really get much in
>the way of protection from lightning by sinking a few extra
>ground rods through the building.

Not sure what you mean by "isolated grounds" - I think you mean more ground rods in different places which WILL increase problems unless they all are fed to a single ground pavilion. But isolation of transducers from unintended ground is important. (Some manufactures get this and use insulating cases if possible. ( There was a time when consumer electronics equipment had metal faces - metal knobs - today everything is plastic or painted to provide isolation - in this case from ESD )

When a transducer can not be isolated, careful planning of the safety-ground path is important. Even ESD can be a problem here - for example - a moisture sensor on a web press gets hit with a discharge from a charge built up from rapidly moving paper - if the ground to the transducer is connected <b>thru</b> the controller board, you will have a voltage drop that depends on the resistance and inductance of that board possibly resetting the controller while paper is flying through the machine(bad things tend to happen).

>As for service entry points having separate grounds for
>different services (electric, cable, phone, etc.), that is
>specifically forbidden by the NEC, and is the reason why a
>system bonding bridge is now required to have a place to tie
>them all together. if people are not building their
>electrical systems to the minimum standards of the NEC, it
>is unlikely any patches are going to work.

Yes - but how many plants are correctly wired? How many have excessive runs to the pavilion? Corroded ground clamps? I've talked to several certified electricians that don't understand why protector grounds need a separate run. ( I once inspected a building fed by two different power companies (for redundancy?) at two different ends of the building - they had problems).


>As far isolation from other grounds, that is forbidden by
>the code, so even if you wanted to do it for some reason, it
>would not meet code.

Isolating what from what other grounds? The ground of the steel structure of a building? Or a water pipe that comes out of the floor to a machine? - I don't have to tie my transducer to them if I don't want to. ( Those two grounds can be at quite different voltages when a building gets hit. )

>I am not sure what you are calling a "ground pavillion". is
>that what the rest of us call a grounding electrode system?

It is called a ground pavilion [single 'l' means a center point ] - it is the point where all the grounds meet - Normally a bus bar in a breaker panel. Your ground rod is supposed to connect there as well. By being physically wide and heavy it should have low resistance and inductance from one end to the other so all the ground leaving the panel should be close to the same potential. ( Doing this right takes care of common-mode problems, but you still have to deal with the differential mode)

> About the only thing substantial
>that is metal underground that isn't typically required to
>be connected to the GES is the incoming gas pipe (in fact
>the code prohibits one from connecting it to your GES).
>Generally there is a fitting at the meter that isolates the
>building from the incoming metal gas pipe (if it is metal -
>most these days are non-metallic).

That is a good example of an isolated ground.

All grounding sources should be thought of as being at different potentials during a transient - we don't want to provide a path from the utility shield wire grounds to a grounded gas pipe.
 
A

aminsameipoor

Dear William

I agree with u. I supposed to explain the problem accurately. sorry for my briefing
So there are additional information :

> 1. What is connected to the +/-12 VDC supply?
there are some Siemens plcs, vibration monitoring system and protection devices.

>2. What electronic devices malfunctioned and how? Were
>they damaged and if so how were they damaged?
i/o cards (Siemens FUM cards have been shown faults in the DCS) and VM600 vibration monitoring system had fault as well. there is not any damages to electronic devices. it was just fault alarm occurred.

>3. Were there similar electronic devices on the -12 VDC
>supply did not malfunction and if so, how were they
>physically related to the malfunctioning devices in the
>power system and ground system?
yes two different systems with different manufacture which had similar problem.

>4. Were there electronic devices not connected to the -12
>VDC supply that were affected by the lightning?
no,there is not

>5. What type of ground system do you have, i.e. a general description?
the ground of here is wide horizontal mesh ground.

>6 How was the -12 VDC supply connected to ground. Does it
>go to an isolated "clean" instrument ground? (a violation of
>the NEC in the US and for good reason).
the -12 vdc is connected to the ground network likewise the lightning arrester and power ground .

>7. Has this happen before? If not, what was different? If
>different, were there any changes in the electrical system?
>Difference in intensity or location of the storm?
yes this was happened three times before.

>8. What protection against lightning or power disturbances
>do you have now?
there is not any protection against the lightning in the distribution system.

>9. What type of electrical ground system do you have if not
>in the US or under NEC?
I do not know exactly the standard code but as i wrote before there is a wide mesh ground and the clean ,power and lightning ground are connected to it directly.

>10. Was there visible evidence of arcing in the system?
no there was not any burning parts .

>11. Were all the devices on the same branch DC circuit?
no just the PLC ,monitoring and protection systems are supplied from this -12/+12vdc.

>12. How were the devices connected to the ground system
>(e.g. equipment ground, case ground, etc.)?
the devices are connected through the ground bus in the electronic room . and that bus is connected directly to the ground mesh pit.

thank a million for your attention.
 
C
I happen to agree with Bob:^) on the predictability of lightning, even NASA uses a lot of trial and error.

Long ago, in a galaxy far away, I worked in an ideal training ground for lightning protection. It was a large hospital campus in Florida. In season, they got hit once or twice a day, often more. You had a thunderstorm every afternoon while I was there. We _had_ to find solutions or fly down once a week to replace equipment. It would even destroy the coax. It looked like a snake eating an egg.

It followed a pattern. We were installing the old Ethernet, in coax. Between nodes. the cable would pass underground or at least under the floor. You would think that would make it immune. We ended up using some potted suppressors that had a BNC in, a BNC out, and a #10 ground wire. I was doubtful they could absorb the energy, but eventually we found the right spots to put them in and the equipment would survive. I never did completely understand this as the coax itself was grounded at each node. But, nobody else had the slightest clue, so I shrugged my shoulders, accepted their kudos, and moved on.

Regards
cww
 
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