TEN350 The waveform that hijacked the IEC lightning standards

The 10/350 waveform is an electrical impulse produced in a laboratory by a surge impulse generator. The "10" refers to the 10 microseconds it takes the impulse to reach 90% of its peak current. The "350" refers to the time in microseconds it takes for the impulse to decay down to 50% of that peak. IEC standards state that this waveform simulates direct lightning and is based on research findings of CIGRE (International Council on Large Electrical Systems - headquartered in France). As shown elsewhere on this website both of those claims are false. Nevertheless, the IEC Class 1 Test stipulates this waveform be used to test SPDs which are to protect against direct lightning.

The mistake we've all made is to think of the 10/350 waveform as a "powerful" impulse. Far more accurately, it should be thought of as an "irrelevant" impulse. A spark gap protector (because it responds slowly and is a crowbar device) can endure a high amplitude 10/350 waveform but doesn't do very well protecting electronic equipment from actual lightning. An MOV protector responds 1000 times faster and actually absorbs energy in the process of clamping the lightning voltage down to safe levels. MOVs don't do so well with a 10/350 waveform because they absorb part of the energy, but they far more effectively protect electronic equipment from actual lightning. CIGRE's 2013 Technical Brochure 549 has corrected the misconception that the 10/350 waveform is the waveform of a lightning first stroke--because it isn't. That is why it's accurate to call the 10/350 waveform irrelevant. We do want stronger SPDs protecting our equipment and that's why we consider it disingenuous for standards to treat MOV-based SPDs as second-class citizens when in fact they are superior at handling direct lightning.

The Lightning Protection Zone (or LPZ) system is a surge protective concept that divides a structure into several "risk zones" nested within each other. The concept has been around since 1977 when E.F. Vance of the Stanford Research Institute proposed it. Here is a diagram showing Vance's risk zones, extracted from his 1977 paper "Shielding and Grounding Topology for Interference Control ." By "grounding" the outside of each shield to the inside of the adjacent shield, Vance sought to control the effect of external surges entering a facility. He also realized the need to limit the surges on the power and data lines entering the structure. Zone 0 was the external environment liable to lightning strikes. Zone 1 was the area inside the structure.

Although the purpose of the LPZ system is to mitigate the impact of incoming lightning, practically speaking, the entire function of the IEC LPZ system has become the regulation of structural and surge protective devices deemed "proper" for use in each zone. IEC international lightning protection standards adopted Vance's idea, but sabotaged it by interjecting the 10/350 waveform. In the IEC 62305 version, direct lightning (Zone 0) must be represented by a 10/350 waveform, hence only spark gap "lightning arrestors" which could pass the 10/350 Class I test were allowed to be used in Zone Zero or at locations bordering on Zone 1 (service entrance locations.) The problems with this approach are documented throughout this web, namely: 1) the CIGRE 2013 Technical Brochure 549 shows that the 10/350 waveform does not represent actual lightning, and 2) the spark gap "lightning arrestors" are intrinsically flawed.

Interestingly, although the IEC-branded LPZ system has been in widespread continuous use for over 20 years, there are apparently no statistical studies to prove its effectiveness. 

More on the LPZ system can be found here.  

In the first case you mention, what's happening is the MOV protectors are responding faster than the spark gaps. That is easy to understand since MOVs inherently react 3 orders of magnitude faster than spark gaps. If the dinky "Class II MOV arrestors" are rated too low to handle lightning (which is always the case with the ones used together with spark gaps) then they can and do burn out before the spark gap can react. As to your equipment burning out, you need to understand that spark gaps may not respond till the voltage level reaches 2.5 kV to 3kV. A transient surge of 2.3 kV is high enough to fry your electronic equipment but not high enough to trigger the spark gap.

Maybe the beer? Marx and Freud also came from around there.

MOVs have a great record at handling direct lightning as you'll see elsewhere on this web. In fact they are superior to spark gaps in this regard because they are much faster to respond and can clamp large currents down to safe levels whereas spark gaps cannot.

Unfortunately not.

You'll have to ask the members of TC 81 about this one. One possible reason is that the 10/350 waveform has always been predominantly a marketing tool and there were vested interests around making sure it was promoted and stayed in place. People who knew there was a major problem with it became afraid of speaking out against it. This could be attributed partly to the shy nature of people and partly to all the force that was employed to keep it going. But these are just opinions.

No. The reason the IEC standards required 3 stages was that spark gaps were unable to clamp overvoltages down to safe levels. They therefore had to be used together with several extra levels of MOV SPDs. A single properly sized MOV-based SPD can itself clamp overvoltages down to safe levels. That isn't to say you would never use additional stages. In critical installations a second stage is typically used as a safety factor and to handle transient voltages that are internally created (i.e. created within the facility itself) or appear on the building grounding steel. The best SPD in the world, if installed at the building entrance, would not be able to forestall damage from the overvoltages of internally-created transients.

It comes from the predilection of many surge protection companies to "dig." The most often heard "excuse" given when spark gaps failed to protect electronic equipment was "Your 5 ohm ground resistance is too high. You need to get it down to 1 ohm for your surge protection to work." This is another urban legend. Per Ohms law, (even discounting impedance which can make this situation worse) a 50kA surge going through a 1 ohm circuit will produce a voltage of 50,000 volts. This is 100 times more than could be withstood by electronic equipment. This only says that no matter how good your grounding is, to protect electronic equipment requires fast acting efficient surge protectors (which eliminates spark gaps.)

There are certainly plenty of folks around who believe that. At this point our opinion is that it doesn't make much difference. The important point is to correct these broken standards with new guidelines that are based on the 2013 CIGRE Technical Brochure 549 and that will effectively protect electronic equipment.

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