ECN Forum Forums Technical Discussion Electrical Theory and Applications TVSS

X

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

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http://www.panamax.com/PDF/IEEE_Guide.pdf

Strange, that is my thought as well.

XtheEdgeX,

The quote you attribute to Mr. Tajali is being used slightly out of context. His comment was made during the discussion of solidly grounded, resistance grounded and ungrounded distribution systems. His point is that low-resistance wye connections make TVSS applications/installations easier, not that a low resistance to ground is required to make a TVSS work.

From the other references you supplied, I get the impression that all of the authors are using the colloquial term "ground" rather than the more correct terms like "grounding conductor" or "reference point".

As a PE I have no problem with the information provided by Dereck.

X

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

Xedge chill out and open your mind for a moment than you are free to respond.

No one has said those PEs you listed are wrong.

What I believe is you are misunderstanding what they are telling us.

For the most part when these PEs say 'grounding is critical' they are not talking about the connection to earth but the interconnection of the EGCs.

The only time a connection to earth may help a TVSS is if it is dealing with a lightning strike.

All other transients must go back to the source which is not the earth.

You are correct there are MOVs from line to ground.

You are mistaken as to which way those are directing current.

Those line to ground MOVs direct transients on the ground to the line and back to the source.

If you tried to direct a utility supplied transient to 'earth' all it would do is find its way back to the source through other grounding electrodes. It is more efficient and of less impedance to direct it right back to line.

Anyway, that has been my understanding of TVSS operation and I have installed a few myself.

A simple question.

Would you say a TVSS installed in an airplane would not work as there is no connection to earth?

Bob



[This message has been edited by iwire (edited 01-03-2006).]

X

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

XtheEdgeX, since you will not post anymore, maybe you might listen for minute.

Functionally there is no difference between Gas Tube, MOVs, and SAD's. They are do the exact same thing, they are all shunt devices. Meaning they appear as an open circuit (or load) until a threshold voltage is reached, at which point they become low impedance devices (or short circuit). In other words they are all shunt devices dsigned to be installed in parallel with the load they are protecting.

The only differences between the three is the amount of energy they can dissapate measured in Joules (Gas having the highest, and SAD's the lowest), the response time in which they operate measured in fraction of a second (SAD's being the fastst, and Gas being the slowest), and robustness or durability (Gas is the most rugged, while MOV's & SAD's are prone to failure from operating). Other than that they operate in the same manner, a shunt.

As I stated earlier in a gounded service N and G are the exact point at the disconnect, they are bonded together. Is that clear? No device type (Gas, MOV, or SAD) can match the performance of a bolted fault. Therefore adding any type of device from N-G or L-G at the entrance can add anything of value to the user. It is a waist of material and money. Only modes needed at a grounded service entrance are L-L and L-N, Ground is already provided by the N-G bond.

Once you are downstream from the entrance then L-G and N-G modes become useful. However earth still has nothing to do with the circuit. The impedance of the EGC and the series impedance of the electrode appear as an open circuit to the fast rise time of a transient. What any SPD does do is clamp or limits the voltage between the two points it is connected too to a safe or reasonable limit. So in the case of a terminal strip TVSS you would select the SPD clamp voltage for the N-G mode to be something like 10-volts, and L-N and L-G to 330-volts.

Dereck Campbell, PE

X

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

Do they make surge protectors for people? No.
Calm down, this is a discussion area. If you want to trade insults, do it with e-mails.
Alan.