SPDs & a ground--needed or not essential.

[lu#26]

I've got a couple of questions regarding surge protection devices (SPDs). Just when it seems that I have a firm grasp of the actual functionality and limits of SPDs, someone writes a white paper or article throwing another wrench in my already tooled-up brain.
I was relieved when I ordered Mike Holt's CD on Grounding vs. Bonding and he basically affirmed what I believed to be true when he said that he didn't understand all the hubbub the SPD industry makes when it insists that an effective ground be present in ones system for a home SPD to work effectively. He said that a SPD basically shunts the surge to the neutral thus making the ground of little consequence. Then I read the IEEE paper on lightning and SPDs where they state:
2.3.1 Grounding
"An effective, low-impedance ground path is critical for the successful operation of
an SPD. High surge currents impinging on a power distribution system having a
relatively high grounding resistance can create enormous ground potential rises
(see Section 4 beginning on page 30), resulting in damage. Therefore, an
evaluation of the service entrance grounding system at the time of the SPD
installation is very important."
As an aside, I overheard two electricians arguing about one of their home PCs being connected to a SPD. Evidently, one of them had an SPD hooked up between his home receptacle and his PC but his buddy had noticed that the ground indication light on the SPD was not illuminated, indicating that there was not an effective ground either at that receptacle or possibly in his entire house. The guy whose PC was hooked up to the SPD shrugged it off saying that he had a UPS plugged into the SPD which, even if the SPD were to fail, the UPS batteries would absorb any surge---Hmmmmmmmmm.
Man-O-man, would I appreciate a little clarity on this topic.

Dan

 

[barbeer]

If the unit is a true Power Conditioner, that should be true, if it is just a battery back-up with surge built in he may be mistaken. I am unsure of how they truly operate so will not offer my opinion on that, BUT, if it were mine I would make sure i had a effective ground.

 

[winnie]

Mostly talking from hearsay here. Use a large chunk of salt:

Most "surge protectors" are 'shunt' devices, which use semiconductor or gas filled devices that insulate up to a particular threshold voltage, and then start to conduct. The idea is that if the surge produces a momentary excess voltage, the surge protection component will create a momentary short circuit, so that the current supporting this surge gets shunted though this momentary conductive path.

The "MOV" is a classic component used for this sort of thing.

An MOV will provide some protection against voltage transients for whatever conductors it is placed between. Good 'surge protectors' place MOVs between all conductors as well as the EGC.

The MOV between hot and neutral will protect from excessive voltage between hot and neutral. This is "differential mode" voltage.

The MOV between the circuit conductors and the EGC will protect from excessive common mode voltage between the supply conductors and ground. "Common mode" voltage is voltage that effects _both_ supply conductors (or both signal conductors on a signal cable). Common mode voltage can often be ignored; think 'bird on a wire'; if the entire bird is at 10000V, there is no shock hazard. Similarly, if your entire computer gets 'bounced' up to 1000V for a moment by a lighting strike, no problem.

A load which is isolated from ground will not care about transient voltages relative to ground.

But if this computer has any sort of ground reference (say a modem, cable modem, network to another computer, or connection to AV equipment), then it will have a ground reference though these other devices. In this case, common mode voltage is a concern. Think bird on a wire holding a grounded twig. ZOT.

If the surge protector doesn't have an EGC connection, then it cannot protect from excessive voltage to ground.

Since a UPS is powered by line and neutral, and supplies line and neutral, then a UPS without an EGC connection cannot provide any protection from common mode surges.

-Jon

 

[lu#26]

Salt goes well with Food for thought.

Salt goes well with Food for thought.

Thanks to both of you. You've got me thinking in the right direction---directly to a SPD manufacturer's website maybe. If I find out anything that will help clear things up more than you already have, I'll post it.

Dan

 

[dereckbc]

Generally speaking earth has no role whatsoever in SPD operation. As stated a SPD is nothing more than a shunt (short circuit when in operation) installed between any two points.

To understand, you must start at the service entrance and understand how a surge enters a building electrical system. In most cases, about 95% according to IEEE, enter via the service entrance.. A typical lightning strike does not hit the building, it hits somewhere ahead of the transformer. Since it takes place on the primary side of the transformer, the surge is in the Differential Mode on the secondary side. On the secondary side, the output is Phase and Neutral, or between the windings.

So your first line of defense is at the service entrance. At the service entrance you need a Class C type device with as low of a SVR rating that you can get. Manufactures play all kind of marketing tricks with the numbers and ratings. What is important is Amps per Mode, and how many Modes the device has. For a single phase service all that is required is 3-modes. One SPD installed on each phase between L-N (2), and one installed L-L (1). Lot of manufacture try to push off 6-modes (L1-N, L2-N, L1-L2, L1-G, L2-G, and N-G) which is a complete waist of material and expense. The reason is because of the N-G bond at the service entrance. No SPD can shunt a solid made bond.

In fact you can even eliminate 1 of the SPD?s to just two modes (L1-N and L2-N) because they are in series and parallel of L-L.. Same thing holds true for poly-phase circuits, you just need 3-modes, rather than the 7-Modes some try to push off.

So for a service entrance Class C device earth or ground has no purpose in operation, only for safety purposes. just like the meter can.

Now once you get downstream from the N-G bond is where things change because common mode comes into play. However earth still plays no role, only the EGC. Look at this way, an EGC is not capable of carrying lightning currents. Let?s say you installed a 750 MCM EGC, and lets say it is only 10-feet long back to the service. At lightning frequencies the impedance of that EGC is in the neighborhood of 27K-ohms. What difference would it make if your GES was 1-ohm or 100-ohms?

What is important downstream is the common mode potential at the point?of-use. Most of all sensitive electronics have components installed between L-G and N-G on the line input mostly in the form of filters for FCC purposes. So it is important to have multi-mode devices installed downstream from the service entrance. The SPD?s will clamp the voltages between all modes to acceptable levels at the point of use. The ground reference may rise up several thousand volts in relation to earth potential, but not between L-N, L-G,, and N-G at the point of use. Those voltages get clamped to acceptable limits by the SPD?s installed in each of the modes.

OK, enough jabber jawing, I think you get the idea.

 

[lu#26]

What is important downstream is the common mode potential at the point?of-use. Most of all sensitive electronics have components installed between L-G and N-G on the line input mostly in the form of filters for FCC purposes. So it is important to have multi-mode devices installed downstream from the service entrance. The SPD?s will clamp the voltages between all modes to acceptable levels at the point of use. The ground reference may rise up several thousand volts in relation to earth potential, but not between L-N, L-G,, and N-G at the point of use. Those voltages get clamped to acceptable limits by the SPD?s installed in each of the modes.

Thanks Dereck. If we were to use the SPD example of the guy with no SPD at his service, a SPD with no apparent ground available (ground light on SPD not illuminated) and a battery backup UPS installed between his computer and his receptacle, is his PC toast if the transformer serving his home receives a lightning strike on the line side?
I live in an apartment with a solid ground, no SPD at the service but have a SPD installed between my receptacle and my PC with no UPS. I feel relatively safe with my setup, or are we both in for a shock!!!

Thanks again for you insightful input.

Dan

 

[ghostbuster]

Excellent info from all of the above.Just to add,for complete protection you must examine every input system attached to your computer systems and spd treat them all.(ie. many damaging surges have been captured coming in via the phone lines for example etc.)

 

[dereckbc]

I feel relatively safe with my setup, or are we both in for a shock!!!

You both could be in for a nasty surprise someday. You get what you pay for.
Let's start with UPS. What most people by for home PC use is a Stand-By type UPS. They are very cheaply made. They basically set there and do nothing until power goes out. In normal mode you PC is powered directly from the line. When power fails a relay activates and connect the battery-inverter. They generate very nasty power and are only made to give the user enough time for an orderly shut down Some have some form of MOV SPD's to varying degrees.

The best UPS's are dual mode operation AC-DC-AC. They are on line and powering the equipment 100% of the time. These are truly isolated from the commercial AC line with lots of protection. To get one though you are going to have to spend 4-digits.

SPD's are in the same boat. This subject can get long, but they come in Class of Service of C, B, and A. Class C devices are service entrance types. They have very large surge current ratings or brute force devices installed as a first line of defense.

What gets through the Class C device is the job of Class B and A devices. A Class B device is ones put in sub panels or branch circuit panel located downstream from the service entrance.

Ok now we down to the actual device you are trying to protect so lets spend a little time with Class A devices aka Point-of-Use. They come in all shapes and flavors in varying degrees of effectiveness directly related to how much they cost. The good ones are what I call Surge Reference Equalizers. Not only do they have heavy duty SPD?s on the line, but also come with SPD?s on all the I/O ports for all your peripherals like coax, RJ-XX, printer, and whatever else. They will have several AC plugs to power your monitor, PC, printer, and gizmos to form a pseudo single point ground for all the SPD?s. So if a transient occurs, all the SPD?s are equalized at the same potential, there fore no discharge paths are present between devices. All the grounds for the SPD?s and equipment are referenced to a Single point Ground Buss inside the box.

The really expensive ones, that I prefer, actually contain an isolation transformer and line conditioners to form a true Single Point Ground, offering complete immunity from common mode potential differences ahead of the device. You will spend several hundred dollars for one of these.

So like I said; You Get What You Pay For.

 

[petersonra]

So like I said; You Get What You Pay For.

And with power protection, you often pay far more than is really necessary. Most people are worried about their PCs getting damaged by a "surge". These days, that is very rare. The power supplies on most modern electronic devices can easily handle the common surges. What they can't handle are real serious ones. So if it bothers you, buy a whole house unit installed in your main panelboard. Don't waste money on UPS's and other bandaids. Keep in mind, most people go many years with no damage at all from surges. They are expensive insurance policies, that rarely pay off.

A typical small UPS can cost you $25-50 a year in electricity just to have it plugged in.

 

[lu#26]

This has been quite an education. I appreciate it thoroughly.

Dan

 

[Pierre C Belarge]

And with power protection, you often pay far more than is really necessary. Most people are worried about their PCs getting damaged by a "surge". These days, that is very rare. The power supplies on most modern electronic devices can easily handle the common surges. What they can't handle are real serious ones. So if it bothers you, buy a whole house unit installed in your main panelboard. Don't waste money on UPS's and other bandaids. Keep in mind, most people go many years with no damage at all from surges. They are expensive insurance policies, that rarely pay off.

Pete
I do not whole heartily agree with that statement. I think it really depends on where one lives.
Where I live, there is quite some density of buildings...hence a lot of wiring in the street. There are storms, acidents (poles being hit) and other related incidents that affect the average home. With that said, I see many people who claim damage to their electronics due to the above mentioned incidents. I say it is not all that expensive if one has expensive electronics in ones home. It is all relative
BTW: electric bills around here average around $300.00 a month... adding $50.00 a year will not be noticed all that much. The bigger homes are much higher than the above mentioned figure.

 

[petersonra]

Pete
I do not whole heartily agree with that statement. I think it really depends on where one lives.
Where I live, there is quite some density of buildings...hence a lot of wiring in the street. There are storms, acidents (poles being hit) and other related incidents that affect the average home. With that said, I see many people who claim damage to their electronics due to the above mentioned incidents. I say it is not all that expensive if one has expensive electronics in ones home. It is all relative
BTW: electric bills around here average around $300.00 a month... adding $50.00 a year will not be noticed all that much. The bigger homes are much higher than the above mentioned figure.

I think you can concoct a situation where it is wise for a residential customer to put a PC or other electronic equipment on UPS or some other kind of power protection system.

IMO, it is not cost effective or appropriate for most residences.

 

[tom baker]

Then I read the IEEE paper on lightning and SPDs where they state:
Dan

The IEEE paper is wrong.
Heres the proof.
Art 285 TVSS was added to the 2002 NEC. Art 285 had a change made for the 05 cycle to allow a TVSS on a delta non grounded electrical system.
A TVSS takes the surge and sends it back to the POCO. G + N connections are for back door TVSS events, in case the G or N voltage is elevated.