600 volt, 3 phase surge protection

[Besoeker]

But there are different classes of SPD's: Some for use at service entrance; some for use at devices inside a home. Their one distinguishing feature is the amount of transient current, they can handle safely

Energy. Joules.

 

[T.M.Haja Sahib]

Energy. Joules.

Carried by current.

 

[Besoeker]

Carried by current.

Volts times amps times seconds.
You need all three for energy.

 

[kwired]

If there was a fault in the SPD, maybe an MOV that fails short instead of open, how much current can the SPD safely handle without catastrophically failing. That is why a SCR is needed. If there is 200kA available on a SWGR for example, the SPD needs to be rated for 200kA in case that amount of current flows through during a fault in the SPD and we don't want it to blow up, just withstand the fault adequately.

I'll buy that. Sounds a little more like a rating for what contains the SPD components more so than a rating for the SPD components themselves though.

 

[mike_kilroy]

from my research, before picking ditek brand, I found that TVSSs are rated in two areas: 1) max joules capability and 2) max amps. My understanding is if you exceed the max amps, it will blow up or if you exceed the max joules (watt-sec) it can absorb it will blow up. so 2 slightly different limits that both have to be considered in an applications.

 

[kwired]

from my research, before picking ditek brand, I found that TVSSs are rated in two areas: 1) max joules capability and 2) max amps. My understanding is if you exceed the max amps, it will blow up or if you exceed the max joules (watt-sec) it can absorb it will blow up. so 2 slightly different limits that both have to be considered in an applications.

Basic electricity 101. Increase the voltage, same resistance = more current = higher watts. I'm guessing the impedance of the TVSS devices is not linear so change in voltage would not be linear to change in current but it still means if voltage goes up, current goes up. If voltage and current both go up so does the watts.

 

[Besoeker]

Basic electricity 101. Increase the voltage, same resistance = more current = higher watts. I'm guessing the impedance of the TVSS devices is not linear so change in voltage would not be linear to change in current but it still means if voltage goes up, current goes up. If voltage and current both go up so does the watts.

They are non-linear devices. Above a specified voltage, they go low impedance thus preventing the voltage from going higher.
It used to be fairly common here to use metal oxide varistors (MOVs) as part of a snubber circuit for protecting semiconductors from overvoltages.
I was never very keen on them myself. That said, we did use Metrosil Silicon Carbide Varistors on a a piece of kit a while back.
It was a static exciter or automatic voltage regulator controlling the field a fairly large alternator. I didn't think it was necessary but the project consultant insisted that it was. To keep a relationship sweet, you sometimes just have to go with the flow.

 

[kwired]

They are non-linear devices. Above a specified voltage, they go low impedance thus preventing the voltage from going higher.
It used to be fairly common here to use metal oxide varistors (MOVs) as part of a snubber circuit for protecting semiconductors from overvoltages.
I was never very keen on them myself. That said, we did use Metrosil Silicon Carbide Varistors on a a piece of kit a while back.
It was a static exciter or automatic voltage regulator controlling the field a fairly large alternator. I didn't think it was necessary but the project consultant insisted that it was. To keep a relationship sweet, you sometimes just have to go with the flow.

To me that means there is no real way to tell what kind of current will flow in a short circuit, if the amount of voltage is not going to be known, and that is what these devices are about - undetermined high voltage conditions.

 

[Besoeker]

To me that means there is no real way to tell what kind of current will flow in a short circuit, if the amount of voltage is not going to be known, and that is what these devices are about - undetermined high voltage conditions.

The overvoltage will be more or less determined at the point where the MOV/TVSS is installed. That's its purpose in life. To prevent damaging voltages getting to equipment.

 

[T.M.Haja Sahib]

The overvoltage will be more or less determined at the point where the MOV/TVSS is installed. That's its purpose in life. To prevent damaging voltages getting to equipment.

It is not called over voltage across the MOV/TVSS, when it is conducting a transient within its rating. It is called clamping voltage.
MOV/TVSS has a clamping voltage across it (in accordance with its rating) lower than the withstand level of the device to be protected, when it is conducting a transient (which should be within its rating)

 

[Besoeker]

It is not called over voltage across the MOV/TVSS, when it is conducting a transient within its rating. It is called clamping voltage.

Well, you can call it what you like.
The simple fact is that the supply voltage has to be over nominal voltage for the the device to conduct.
Simple. Innit?

 

[Besoeker]

After successfully interrupting the transient current through it, the SPD

That's another point. The SPD does not interrupt current. It sinks it.
The current ceases when external conditions no longer drive it.
External conditions like the voltage returning to normal after an overvoltage transient.

 

[T.M.Haja Sahib]

I meant by

After successfully interrupting the transient current through it,

the SPD did not allow the power frequency current to flow through it, thereby making a short circuit.

 

[kwired]

That's another point. The SPD does not interrupt current. It sinks it.
The current ceases when external conditions no longer drive it.
External conditions like the voltage returning to normal after an overvoltage transient.

Exactly why I am questioning just what this short circuit current rating for these devices is all about.

Amount of current that flows through them is dependent on the voltage of transient and how much power is behind the transient. That is not a short circuit, that is the device doing what it was intended to do. If energy level is high enough catastrophic failure should be expected to happen.

At normal system voltages the thing should be passive so that is not going to contribute other than if there has been a catastrophic failure, only then can I see there maybe being an issue of whether or not the enclosure for the device to be able to contain the event, if the energy of the transient didn't destroy the enclosure.

 

[Besoeker]

I meant by

After successfully interrupting the transient current through it, the SPD

the SPD did not allow the power frequency current to flow through it, thereby making a short circuit.

It isn't a short circuit.
It's has a variable resistance characteristic. Hence varistor.

 

[Besoeker]

Exactly why I am questioning just what this short circuit current rating for these devices is all about.

Amount of current that flows through them is dependent on the voltage of transient and how much power is behind the transient. That is not a short circuit, that is the device doing what it was intended to do.

I agree - it's not a short circuit.

If energy level is high enough catastrophic failure should be expected to happen.

Again, I agree. Ultimately, failure is because of excessive energy dissipation.

 

[mike_kilroy]

I agree - it's not a short circuit.

Again, I agree. Ultimately, failure is because of excessive energy dissipation.

I am sorry but that is a nonsense answer as would be given by a non engineer without a basic understanding of physics.

of course there is a MAX CURRENT rating as well as a MAX POWER rating. Either one can destroy a device.

 

[kwired]

I am sorry but that is a nonsense answer as would be given by a non engineer without a basic understanding of physics.

of course there is a MAX CURRENT rating as well as a MAX POWER rating. Either one can destroy a device.

Can you answer my question of where does the short circuit current come from that has been mentioned earlier that surge protective devices must have a rating for?

If there is no over voltage condition these are passive. No current to carry or interrupt. If there is an overvoltage condition they begin to clamp current. The more overvoltage the more current. This is still not a short circuit or at least not a very predictable amount of current will flow, but could be pretty high current. It is much easier to predict how much current will flow when you short two leads together based on the size and impedance of the source plus throw in impedance from there to the short location. How are we supposed to know how much voltage there will be during a transient condition and how much current the device may need to carry when transients are pretty unpredictable - especially those caused by lightning?

 

[mike_kilroy]

the MAX current rating on TVSS devices comes from the manufacturer. they test them and find at what current level they fail. I dont believe I every called, or any mfgr specs I have ever seen called, this max current level a short circuit level. Surely this max current rating comes from what point the wires will vaporize or the physical semiconductor material of the TVSS nearly instantaneously blows apart. Of course the max joule rating would come not from this instant max rating but from a time rating after which the generated heat melts the semiconductor material. both ratings are very real limits.

 

[Besoeker]

I am sorry but that is a nonsense answer as would be given by a non engineer without a basic understanding of physics.

of course there is a MAX CURRENT rating as well as a MAX POWER rating. Either one can destroy a device.

That infers you can have them independently. You can't (except maybe in the case of a superconductor).
The device is destroyed by heat which is energy.
Current times voltage times time.
All three are required.
That's just basic physics.