ECN Forum Forums Technical Discussion Electrical Theory and Applications TVSS

X

[This message has been edited by XtheEdgeX (edited 01-16-2006).]

X transients arriving via the service is a differential mode rather than common mode, meaning the voltage is L-L and L-N. Earth impedance is of no value or importance, it is merly a reference point.

"X" as a moderator at ECN I must ask you to edit your post or have it edited for you.

Profanity is not tolerated here at all.

Spirited discussion and disagreement sure.

This web site is strictly rated "G".

Bob

There is a certain amount of truth to that, but turn it around.

Because we all know Dereck we have a fair idea of the depth of his knowledge.

We don't know anything about XtheEdgeX and frankly, right or wrong that name makes me think of skate boarder.

So far what sticks with me is your quickness to fly off the handle.

Bob

Dereck: X transients arriving via the service is a differential mode rather than common mode, meaning the voltage is L-L and L-N. Earth impedance is of no value or importance, it is merly a reference point.

So you can then look at it as the equipment sees it: a connection to 3 points that are labelled,"L", "N", & "G". We know that the other ends of "N" & "G" were at the same potential at a main panel and that the equipment ends may or may not be. But if we look into the equipment for any components between N & G, there aren't usually many susceptable ones there. You might find a few HF bypass or decoupling caps or maybe a spark gap or two. But in the over 25 years that I have repaired a broad range of equipment, I have rarely had to replace components L-G or N-G. My forgetter is getting as good as my rememberer so it's possible I'm forgetting a few HV discs I replaced in the 80's and just remember the half dozen or so input bridges and switching mosfets I've replaced in the past year.

If I'm given a hypothetical choice to use 2 components to protect my gizwalter, I'll pick a bi-directional transorb and a fast acting fuse. The closer the clamp voltage above the highest L-N expected, the better. So once in a blue moon, your mains drift up and you pop your fuse. So what! My fuse is not going to be much higher than my max I and definately picked to blow before the surge and continuous ratings of the transorb are exceeded. If I'm allowed a 3rd component, I would pick series inductance in the line after the transorb before I would clamp L-G or N-G. I think any good electron lawyer would claim that surges delayed are surges denied. Scott posted several drawings of surge arrestors that were just pi-type LC input filters with MOVs in parallel with the caps.

I'm curious if other folks out there who have to fix things after they blow have had similar experiences with the components that they have had to replace.
Joe

Regarding the TVSS discussion taking place:

As mentioned previously, the intents of many TVSS devices installed on the Load side of the Main Disconnect, are to "Divert" Surge(s) from the "Active Circuit", with hopes that the devices connected to the normal Active Circuit (L-N) will not have to deal with any surges.

The idea is to attempt to pass surges, which originate from sources both external and internal to the system, around the sensitive load item(s), and back to the Power Supply.

Except for Lightning surges and RFI (or even line charging...but that's stretching it!), the surges will be between Source and Load - or Transformer and Equipment.

This would mean that any TVSS component connected to the "Grounding Conductor", outside of the Point Of Physical System Earth Bonding, is simply shunting the surge load around the normal active circuit, back to the Point Of Physical System Earth Bonding - and eventually returning to the Power Supply of origin.

BTW, Point Of Physical System Earth Bonding is in reference to the location on a given AC Power System, where the System's Grounded Conductor is Bonded to the local Grounding Electrode System - via the Grounding Electrode Conductor(s); along with the Metallic Enclosures and Equipment Grounding Conductors being Bonded to the same "Star Point".

Surge Protection Devices (SPDs) on the "Line" side of the Point Of Physical System Earth Bonding, will still - in effect - be connected the same as others - unless they are not connected to the same Electrode as the AC System - and this Earthed side is isolated from any physical connection to the Bonded AC System's "Grounding System" (GES and Enclosures).

In our field, the term "Grounding" and "Grounded" are very misleading - and at times downright confusing; to the point of pending disaster!

I encourage using terms like "Bonded", "Bonding" and "Equipment Ground Bonding", in the field, so the idea of a physical connection to the AC system is made - instead of the notion of several mysterious connections of wires to the Earth.

There is (typically) only one connection of the AC system to the "Earth Ground", and this is only to establish a "Neutral Point" - not for any system performance reasons.
The system will work exactly the same with no physical Earth connection - and this includes TVSS devices (as long as the EGCs still terminate to a given Conductor of the referenced AC system).

I need to create an easy to follow seminar of basic system grounding principles - how, what, where, when and why scenarios to describe the ideas of AC System Grounding, and submit this to the Technical Reference section.

This is really important, and once again, I have had to resolve some major confusion of Grounding Techniques in the Field - between Clients, Contractors and our own Employees.


Per a part of "Joe Testengineer's" last message


This is 100% correct! And goes along with the first part of my reply.

The TVSS designs I submitted on-line (posted in the Technical Reference section), are - by the most part, simple PI filters - with MOVs in Parallel (between L-G and N-G).

The intent of these TVSS devices are to shunt surges away from the Load devices, by "Dumping" the surges into the Equipment Grounding Conductor, and "Sending Them" back to the Source / Supply via the connection of the system's Grounded Conductor to the Bonded Equipment / GES Star point.

On Ungrounded AC Systems, the termination of TVSS devices, along with Low Pass / Band Pass filtering terminating to an Equipment Grounding Conductor, will still eventually result in flows returning to the Power Supply (Transformer).
They will be limited to the level of flow which would be Capacitively Coupled to the System.
Or in the cases where the Secondary side of the PoCo's Transformer is Earth Grounded via a Center Tap point only (and there is no system Grounded Conductor), there will be a flow through the Earth Ground, between the local Grounding Electrode System(s) and the PoCo's Transformer Earthing Conductor point of connection to the actual Ground.
I will include a few drawings and examples + explanations of this type of Ungrounded AC System.

OK, time to leave the soapbox!

Scott35

Joe, I agree with your assessment. To summarize L-G, N-G modes are not necessary at the service entrance, and earth ground has no real function in the operation of a TVSS device. Earth is just a reference point and is used for safety. Down stream from the entrance N-G and L-G modes become useful because a lot of sensitive electronic equipment has components like caps, mov’s, and RFI filters installed in these modes for FCC requirements. Again, the earth ground has no function. However, the EGC does become a factor b/c if we were to have a surge current flowing in the neutral circuit and/or EGC, this would cause an excessive voltage drop between N-G, therefore a SPD between the 2 would limit the potential difference to a safe operating limit felt across the equipment terminals

Here is a passage of what I have written in the Emerald Book:
“Low-voltage end-user type surge protective devices are often described as transient suppressors, but their operation is really a diversion of the surge current through a low impedance path preventing the rise of high voltages across the load terminals. For large surge currents, this diversion is best accomplished in two stages. The first diversion should be performed at the entrance to the building, typically by conventional surge arrestors rated for this duty (a class “C” device). Then, any residual voltage resulting from the action of the arrestor can be dealt with by a second protective device at the power panel, or at the terminals of a connected load (a class “B” or “A” device) . Note that class “A” devices are sometimes called point of use devices.”

SCOTT, I looked at your drawings and correct me if I am wrong. Your devices appear to be class “A” devices aka Point of Use devices. I agree the LC network would dampen any transient on the load side, and recognize the value of the N-G and L-G modes. These would great devices on the service side except you would not need any of the N-G and L-G modes. But the problem is they would have to be service rated equipment and very expensive to pass service current ratings of 100 + amps. In theory I know they are possible, but I have not run across any in my adventures. In fact your devices look a lot like the ones used in Triplett Surge Reference Equalizers for Point Of Use devices. A very fine product indeed.




[This message has been edited by dereckbc (edited 01-18-2006).]

OK, I obviously started something here, and I will try and make ammends. With TVSS, I was taught that in order for it to function properly, it had to have a good low impedance path to ground to deal with any excess voltage that the clamping device didn't take care of.
I'm sorry for anyone who may have gotten a bad impression of me. Sometimes I get a little stubborn about my work related stuff. If you were to be debating something like this with me in person, and I love to debate, you would see that I will convey my opinions this way, but actually not be upset, as it may have seemed. Kind of like ribbing a co-worker. For example, if I was talking about this with one of the guys at work, he may say to me "Shut up, you don't know what you're talking about". And I might say "Who do you think you are? You're just an apprentice". LOL. Does that make sense? But, when I sat back and read my posts, I saw that some things could have been offensive. I apologize for that. It's hard to convey feelings like that when you write it down. Anyway, I'll try and get on the right foot, and choose my words more carefully. I would also like to add, that if I have been taught a way that someone doesn't think is correct, I won't change my thoughts just because someone tells me I'm wrong. I would appreciate it if I could be directed to something more convincing. If I say this is how to do this, and you say no it's not, look at NEC section such n such, I will see that I was wrong. Or direct me to a IEEE standard or an OSHA reg or something. I've encountered too many people that would show someone the wrong way, that's all.

X, no problem, lets move on. Think about this for a while. Let’s go back to your assumption that a low impedance earth ground is a factor, before I do that, some good reference material is IEEE STD 1100-1992 (aka The Emerald Book POWER GROUNDING SENSITIVE ELECTRONIC EQUIPMENT). I am co-authoring the 2006 release and have some insight. Also NFPA -780 is excellent material.

Let’s assume we go to the trouble of installing a ring ground with chemical rods and get that elusive 5-ohm or less ground. Now we bond our service with a lot of overkill using a 10-fott long piece of 750 MCM ground electrode conductor. Most people would think this is a killer system no lightning could damage. Would you agree?

Now let’s apply some basic physics and electrical principals. Lightning and surges are high frequency events so inductance and impedance comes in to play. So let’s assume the event is lightning and it hits the weather head, the worst case scenario. Due to the very fast current rise time of lightning the impedance of that 10-foot long 750 KCM cable becomes a few kilo-ohms, for our example let’s say 2000 ohms. 2000 ohms is in series with our killer 5 ohm ground electrode system would be 2000 + 5 = 2005 ohms at the N-G bond point. Do you see that logic?

If so, I now pose the question to you. What difference does it make if the ground electrode system impedance is 5-ohms or say a two rod 50-ohm system? If it were a 50-ohm system the N-G bond point would be 2000 + 50 = 2050. Is there any significant difference between 2005 and 2050 ohms? Now for the reality bites section; The 5 or 50-ohm ground electrode measurement is measured at DC or power frequencies which have nothing to do with High Frequency (HF). At HF the impedance would be many magnitudes higher depending on the frequency of interest.

Now for a final thought or two. If lightning were to hit the weather head as we used in our example, the voltage at the N-G bond would be several thousand volts above local earth ground. Let’s just say 10,000 for argument sake. Although the reference ground point (the N-G bond) goes to 10,000 volts, so does our Line and neutral voltage. The job of the TVSS is to clamp the voltages between terminal points to a survivable level. Since N-G is bonded by a bolted connection that mode and L-G mode is covered at the service entrance only. All we have to do is clamp is the L-L and L-N modes that will be felt by the terminal equipment. If we clamp the L-L to say 400 volts, and L-N to 330 volts we should be OK. Yes the voltage would rise to 10,000 on all circuit conductors with respect to local earth ground but not between the EGC-N-L that the equipment is seeing.

Do not going away thinking a low impedance ground is not important, because it can be when we are talking about lightning protection systems. We will save that topic for another time.

Hope that helps.

Dereck

X glad to have you back, I did not mean that you had to pull all your posts, only the words you would not use with children.




A stubborn electrician.....what a shocker.

I certainly am guilty of that.

Not many of us here are not the same way, it seems to go hand in hand with pride in the job we do.

Anyway the past is history, today is a new day.

Bob