ECN Forum Forums Technical Discussion Electrical Theory and Applications TVSS

Hello “dereckbc”:

You are confusing the “common mode” term with the “normal mode” one. “Normal mode” suppression is “somehow” present in the “common mode”, can you see why? Draw the diagram. But I’m sure for most commercial TVSSs, this “somehow” I’ve mentioned set us outside the IEEE 587 guidelines.

Second, the only point in which NG protection is irrelevant is very close to the N-G bonding point, but only as long as that bonding remains intact. Depending on the needs it could be necessary to have NG protection even for Class C TVSSs (most integrated units I have seen include it).

United Power has a simple yet interesting article about this topic: http://www.unitedpowercorp.com/technical/ModularSurge.htm.

I’m wondering myself if being so “definitive” as you were could drive someone for the wrong way. Maybe we should be more careful with our comments. It could be better to point the direction instead of dictating the solution.

Regards,

Joe.-

Joe of NJ, don't mean to step on your toes, just trying to de-myth manufactures claims. I know very well the difference between common and normal modes, and at a service entrance where you have a MBJ there is no common mode, or better said comon mode and normal are the same...

I have drawn it out and designed TVSS units for GE. Here is a link of some white papers I published that relates directly to common mode and normal mode.
http://www.geindustrial.com/products/applications/service_entry.pdf

Here is a quote:

There are no real benefits of choosing a 7-mode TVSS (asuming 3-phase service) over a 4-mode TVSS at service entry when the N & G are bonded except for allowing a manufacture to quote "per phase" maximum surge current ratings.

The rest of the publications can be found at http://www.geindustrial.com/cwc/pro...p;id=tvssoem&typeId=2&lang=en_US

Let’s see, common mode transients are those occurring between lines and ground (LG and NG). Yes, neutral is a line, but if we don’t want to consider it a “line” then we have to differentiate two kinds of common mode transients, the ones between lines and ground (LG) and the ones occurring between neutral and ground (NG).

Normal mode transients are those occurring between each line and the others (LL and LN). Again, if we don’t want to consider the neutral as a line we should recognize two kinds of normal mode transients, the ones between lines, and the ones between lines an neutral.

At the system bonding jumper, neutral and ground became one, so in both cases (considering the neutral a line, or not doing so) the transients could appear between lines and this neutral-ground entity (LN = LG, common mode transients) or between lines (LL, normal mode transients). I don’t see why you insist in the non-existence of common mode transients.

By the way, normal mode transients may be called transverse mode transients or differential mode transients.

The paper you wrote, shows the difference between 7 and 4 devices TVSSs, both of them in common mode configuration. In your paper you show why when neutral and ground are bonded the use of a 7 device TVSS is equivalent to the use of a 4 device one. That could be theoretically correct, but that has nothing to do with normal mode versus common mode (or, if it does I don’t see it).

We have to remember that according to NEC 250.30 A 1, the system bonding jumper “ …shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, ….”. So, there is no assurance that at the Class C TVSS the neutral-to-ground path is 0 ohms (or something equivalent) at the transient spectrum and considering the surge current capabilities. 300 amps only require an impedance of 1 ohm to produce 300 volts! I could agree with you in that for Class C TVSSs installed at service entrance the NG TVSS device could not be of much help in many applications; but I disagree with the idea of it being totally unnecessary in all kind of situations.

But all this is not too much critical at least we are thinking on individual TVSS devices (individual, 2 leads elements), because even those GE offers in the website you pointed at shown NG protection. I know, that protection is given by the series of TVSS devices between LN-LG, but anyway they are promoting it, so I’m afraid you need to keep working in your de-myth project, to make it really transparent (and not only concentrate in de-mystifying other manufacturers products while hiding your own myths [by the way, GE is not the only one doing this, most manufacturers may be doing the same]). So, if it is not possible to evade this NG protection, why bother arguing it is not necessary and it is a waste of time and money?

Please, this is not personal, I don’t want polemics, but as I stated before, we should be more careful with that solution-designing feeling that want to control us from time to time.

Joe.-

Yins guys are sounding pretty high brow so I might as well jump into the fray. One thing I think I've learned is that no matter how many times I tell lightning how it's supposed to behave, it never seems to listen. Ponder my thoughts and feel free to blast away at me. I won't feel hurt.

Lighting strike = single turn primary??

Area between lightning and house = air core??

House wiring roughly parallel to strike = loosely coupled, single turn secondary??

Main panel surge suppressor = sitting there whistling, doesn't know that the furnace ctl. board just blew up, again. Doesn't know that it's going to get blamed for not stopping a surge that didn't come from the utility.

OK, blast away, make it pithy & no bloviating. ( I'm watching waaay too much O'Reilly.)
Joe

Joe of NJ, please consider these items.

1. According IEEE 95% of all surges happen on the primary side of a transformer in the common mode. If that is the case then the surge appears on the secondary as a differental or transvers mode (L-L & L-N

2. At the service entrance the neutral and ground are bonded together. There is no SPD that can equal that short circuit. If you were to add a SPD between L-G or N-G would only parallel the L-N device. Therefore adding devices L-G and N-G anywhere there is a N-G would be waisted material and ineffective. You get more bang for the buck by using a larger SPD device than adding multiples in parallel.

3. When you bond the N-G you essentially make the differential mode and common mode one in the same becuase they are both the same point electrically at the N-G bond. I never implied that transverse and common were the same, only at the service entrance. Please go back to my first post where I sated all-modes were usefull downstream of the N-G bonds, about 80-feet to be exact.

4. Common modes does become important downstream from the entrance and downstream from SDS for the Class B and A units (sometimes called Point of Use). But earth in no way has anything to do with the operation of a TVSS which was my original point. Joe test, that should address your concerns as of the induced strike.

X

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

XtheEdgeX


No that is not what they are installed to do.

What you have is a basic misconception here. Voltage is not 'dispersed' to ground. It must be directed back to the source, the earth is not the source of the transient voltages.

Voltage does not 'want to' go to ground it 'wants' to go back to the source.

The source will be (most times) the first transformer on the supply side of the TVSS.


[This message has been edited by iwire (edited 12-31-2005).]

X

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

Sorry I still think you are mistaken as to what the earth will do for a TVSS.

Please re-read Dereck's posts, he is a very knowledgeable man.

Bob

X

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