600 volt, 3 phase surge protection

[ron]

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?

Just like busway, transfer switches, safety switches, etc, the manufacturer indicates a withstand rating (short circuit rating) for devices that are not designed to interrupt current. A short circuit can occur in an SPD just like any of the devices indicated above. The SCR It is not directly connected to the ability for the SPD to shunt transient overvoltages to ground.

 

[Besoeker]

Just like busway, transfer switches, safety switches, etc, the manufacturer indicates a withstand rating (short circuit rating) for devices that are not designed to interrupt current. A short circuit can occur in an SPD just like any of the devices indicated above.

The short circuit being from where to where?

 

[kwired]

The short circuit being from where to where?

I'm with you so far on this.

Best I can see is any required short circuit rating must be for the ability for the device enclosure to contain what may happen at that rating if the device should fail under normal supply voltage. Amount of current that may be involved in a transient is not as predictable as amount of short circuit current can be supplied by a known source. These devices are otherwise designed to fail if the transient is high enough level of energy.

 

[topgone]

The short circuit being from where to where?

If we define what a short circuit is, it doesn't fit its definition. SPDs clamp the voltage by conducting to the ground during overvoltage events/ spikes. The amount of current depends on the amount of overvoltage and the dynamic resistance of the semiconductor.

I guess we have to agree that it is the excessive current and the extended time said current flows in the SPD that will destroy it; I²R x time, that is.

 

[ron]

The short circuit being from where to where?

For any phase to phase, phase to ground or phase to phase to phase short, just like any electrical piece of equipment. It is unrelated to clamping voltages and such, but if there is an internal fault when maintenance is occurring, rodent entry, internal component failure, etc. the device has to be rated and braced to not catastrophically fail.

 

[Besoeker]

For any phase to phase, phase to ground or phase to phase to phase short, just like any electrical piece of equipment. .

But that wouldn't be through the SPD unless, of course, it had already failed.

 

[ron]

But that wouldn't be through the SPD unless, of course, it had already failed.

Yes, after a failure. Somewhere inside fails.

 

[Besoeker]

Yes, after a failure. Somewhere inside fails.

Let me see if I've got this right.
A device with a short circuit current rating that isn't a short circuit until the device actually fails??

 

[ron]

Let me see if I've got this right.
A device with a short circuit current rating that isn't a short circuit until the device actually fails??

True.
Similar to busway, safety / disconnect switches, etc.

 

[Besoeker]

True.
Similar to busway, safety / disconnect switches, etc.

Not similar at all.

Disconnectors, circuit breakers, isolators, busbars etc are rated for fault currents that could arise from faults external to them. You don't really expect busbars to catastrophically fail in such circumstances.
In fact you design them to withstand calculated prospective fault current. It isn't their failure that results in fault current.
In the case of the surge suppression device, you need it to fail to result in fault current.
Not just that - you need it to fail in a particular way.

 

[ron]

Not similar at all.

Disconnectors, circuit breakers, isolators, busbars etc are rated for fault currents that could arise from faults external to them. You don't really expect busbars to catastrophically fail in such circumstances.
In fact you design them to withstand calculated prospective fault current. It isn't their failure that results in fault current.
In the case of the surge suppression device, you need it to fail to result in fault current.
Not just that - you need it to fail in a particular way.

I think we are mincing words. I've seen plenty of failures inside busway and disconnect switches that needed to be contained and the manufactured design internally of them to withstand calculated prospective fault current was important to not become catastrophic. Leftover tools and rodents cause lots of internal failures in equipment.

 

[Besoeker]

I think we are mincing words. I've seen plenty of failures inside busway and disconnect switches that needed to be contained and the manufactured design internally of them to withstand calculated prospective fault current was important to not become catastrophic. Leftover tools and rodents cause lots of internal failures in equipment.

Yes, I've seen that too. But a fault rating of say, a 50kA busbar system, is to cope with external faults.

Copper busbars are normally part of a larger generation or transmission system. The continuous rating of the main components such as generators, transformers, rectifiers, etc., therefore determine the nominal current carried by the busbars but in most power systems a one to four second short-circuit current has to be accommodated. The value of these currents is calculated from the inductive reactances of the power system components and gives rise to different maximum short-circuit currents in the various system sections.

For the surge suppression device fault current flows only if the device has failed in the first place.
And then only if the nature of the failure IS a short circuit.
Most I have seen have been blown apart and the result is then an open circuit.

But I take your point. If it does become a short circuit, the failure needs to be contained.

 

[mike_kilroy]

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.

nope, not correct. yes, you can have hi current and low joules. either one can kill the device so both are important. Just look at any TVSS device spec - for example quick digikey search:

http://media.digikey.com/PDF/Data%20Sheets/Thomas%20Research%20Products/BSP3%20LC%20Series.pdf

pick the 480v model for an example. good for 360 joules. also limited to 10,000amps max amps. industry test appears to be 8/20 usec pulse, so 550v*10,000a*8/20E-6=2.2 joules. MUCH less than its max heating power rating. But it will blow up if we hit it with 20,000a*550v*8/20E-6= 4.4 joules.

Consider that 99% of all lightning strikes are less than 55kamps. this device would not be a good choice for a home service entry at main cb box, even tho it shows a substantial joule rating, but a 100,000amp surge one would.

 

[Besoeker]

nope, not correct. yes, you can have hi current and low joules. either one can kill the device so both are important. .

Let me remind you of what you posted earlier:

"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."
For the wires or the materials to vaporise they need to get very hot. For that to happen requires heat.
Heat is energy. Joules.
Volts times amps times time.

 

[mike_kilroy]

OK, so maybe a good way to see why a max surge current limit is BY ITSELF is a real limit IN ADDITION to a joules limit would be with my examples above stated slightly differently.

It is like a typical car warranty: A or B - whichever comes first.

Take the same device as above, 360 joules AND 10,000amps max limits....

scenario 1: spike comes thru that amounts to 359 joules & less than 10,000amps: device lives.

scenario 2: spike comes thru that amounts to 1 joule & 10,000amps: device lives.

scenario 3: spike comes thru that amounts to 4.4 joules & 20,000amps: device dies.

 

[Besoeker]

OK, so maybe a good way to see why a max surge current limit is BY ITSELF is a real limit IN ADDITION to a joules limit would be with my examples above stated slightly differently.
scenario 3: spike comes thru that amounts to 4.4 joules & 20,000amps: device dies.

What is the failure mechanism?
What exactly causes it to vaporise?

 

[mike_kilroy]

I dont know, have not researched the failure mechanism; i dont mfgr these, i have other things that occupy my time. but all i am reporting is that there IS a max surge current limit that ALL mfgrs of surge protection devices lists as max values that if exceeded will cause their devices to fail. this tells me that those who care about the exact failure mode HAVE studied it and know where their limits are; Joules for heat failure and max amps for another failure. Obviously it is not a heat issue as joules is but rather a more instantaneous failure mode. I am sure you have witnessed diodes or transistors blow up 'instantaneously' from an applied voltage over their max PIV rating? 200, 300, 400 amp devices sound like shotguns going off - and most of the time there is not enough energy in the hi voltage spike to cause this sound or resultantant device parts blown to litter, so it is another type failure mode - exceed PIV sufficiently and the semiconductor device shorts, NOW there is full 460v or whatever available to it to violently blow. That is the picture I had in my head when I suggested in earlier post that it is probably a more instantaneous failure mode when only a couple joules of energy can fail it. sorry i mispoke and implied heat. the fact remains that there are at least 2 limits to MOV/TVSS type devices, watt-sec AND max amps; i was simply trying to point this out to those who repeatedly said max surge is not a limit.

 

[Besoeker]

I dont know, have not researched the failure mechanism; i dont mfgr these, i have other things that occupy my time. but all i am reporting is that there IS a max surge current limit that ALL mfgrs of surge protection devices lists as max values that if exceeded will cause their devices to fail.

Surge current is not the same as short circuit current.

Joules for heat failure and max amps for another failure. Obviously it is not a heat issue as joules is but rather a more instantaneous failure mode.

It's a heat issue whichever way you cut it.
V*I*t.
You simply cannot treat them as independent variables as you seem to have done.
That's basic physics.

I am sure you have witnessed diodes or transistors blow up 'instantaneously' from an applied voltage over their max PIV rating? 200, 300, 400 amp devices sound like shotguns going off

If you had ever examined the nature of the damage to the silicon chip from a failed large diode or SCR, you would have observed that the damage, whatever the mode of failure, is some or all of the chip had melted. You can sometimes determine the cause of the failure by the position and area of the chip that melted. But melted it has. And melting needs heat.
It really is that simple.

 

[ELA]

I believe you are discussing the difference between a lower wattage heating effect and a very high wattage heating effect. In the later the heat is very intense in one small area.
A term often applied to semiconductors is "localized hot spotting".


http://www.iaei.org/magazine/2004/03/metal-oxide-varistor-degradation/

If you have limited time jump down to "Degradation of MOVs.

 

[Besoeker]

I believe you are discussing the difference between a lower wattage heating effect and a very high wattage heating effect. In the later the heat is very intense in one small area.
A term often applied to semiconductors is "localized hot spotting".

I agree. The hot spot is still a thermal issue. Too many joules in a given area.