TEN350 The waveform that hijacked the IEC lightning standards
Real lightning protection exists and is effective.
GOOD NEWS. We would be remiss in our responsibility if we allowed you to get to the end of this website thinking there was no such thing as effective lightning protection.
The fact is the 2013 CIGRE Technical Brochure 549 validates effective surge protection strategies and devices that have been successfully protecting electronic equipment for 5 decades. Now that CIGRE has identified the actual dangers posed by lightning, it is not a difficult task to specify surge protective measures capable of keeping electronic equipment safe from those dangers.
The key to effective surge protection is providing an alternative low impedance path to ground which lightning will take in preference to the path it would otherwise have traveled (straight through your electronic equipment.) In almost every location, out of 100 dangerous events fewer than 4 will be the result of direct lightning strikes to the structure. The rest will be caused by the indirect effects of lightning being brought into the structure via the connected power or telecommunications lines. Direct strikes are likely to be the most deadly and structures subject to them should be protected with lightning rods, down conductors, and earthing systems. To protect against the far more prevalent type of danger (the surges entering the building via the ground or service lines) surge protectors must be installed. The surge protector at your service entrance must be rated to handle 160kA lightning current (8/20 µs) and 200kA - 300kA for critical applications or locations with high lightning incidence. The surge protector must be fast -- able to divert large currents in just a few nano-seconds. That way, rising surges get clamped fast enough to keep the downstream electronics safe. An SPD must be designed to be able to handle multiple impulses - the most characteristic of all lightning parameters. And an SPD's viability must be commensurate with the design life of your application.
What follows is a chart showing the key lightning parameters identified in CIGRE's 2013 TB 549 together with the relevant applicable surge protective specifications:
PARAMETER FROM CIGRE 2013 TECHNICAL BROCHURE 549 |
APPROPRIATE LIGHTNING PROTECTION SPECIFICATION |
| 5% of all lightning can be expected to be higher than 100kA (Sect. 3.1) | Specify a minimum surge rating of 160kA for service entrance. |
| Highest direct measured peak current is 200kA for negative lightning and 300kA for positive lightning. Inferences from remotely measured electric and magnetic fields suggest the existence of currents up to 500kA (Sect. 3.1) | For critical applications or high-lightning environments, specify an SPD rated 200kA-300kA. |
80% of all lightning flashes contain multiple impulses. (A first stroke and average of 2-4 subsequent strokes.) ; Average time between intervals: 60ms. |
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| In 30% of cases subsequent strokes have larger currents than the initial strokes. | Specify multiple individually-fused protection paths for each phase. This kind of true redundant design provides your equipment with continuity of protection even if an SPD component is damaged by a first stroke. |
| Voltage drops (surges) can occur between live conductors, between conductors and neutral, between conductors and ground, and between ground and neutral. | Specify surge protectors with "all-mode" protection. |
Sect. 10.2 of the CIGRE report tells us that the density (number) of lightning ground flashes at a certain location "is by far the most relevant factor in any kind of application related to lightning protection." Lightning strikes the earth 25 times per second, every second of every minute of every hour of every day all year every year. |
Won't lightning waves disperse or diminish after hitting a structure or electrical system? Is an SPD rated 160kA, 200kA or higher really needed?
Even though it is unlikely that an SPD will ever see a surge of 200kA, such ratings are necessary. Here are five reasons:
1) Handling real lightning. The CIGRE 2013 report confirms that more than 80% of lightning flashes consist of multiple closely-spaced impulses (average 3-5). If an SPD cannot handle that aspect of lightning it should not be called an SPD. This is what the spark gap manufacturers don't want you to know: a poorly crafted SPD that might pass a single impulse test, would yet be totally incapable of handling the multiple impulses of actual lightning. MOV-based SPDs rated 160kA and above are robust enough to handle this "multiple impulse" aspect of lightning that has been up to now completely ignored by the 62305 lightning protection series. (Read how the multiple impulses of real lightning create far greater stress on SPDs than a single impulse.)
2) Clamping power. SPDs constructed with the ratings recommended in the above chart can share surge current between SPD elements and have much better clamping power. For example we know of a test where a single MOV-based SPD clamped multiple 100kA surges to levels low enough to protect sensitive electronic equipment.
3) Fusing. SPDs so-rated can have built-in fusing of 200kA to 300kA. This is far superior to dinky SPDs with a single 20kA or 40kA fuse. An SPD with 200kA multiple-fusing will handle more and larger surges without failing.
4) Viability--SPDs with the ratings shown in the above chart will last a long time. If your home or cell site or factory was built to last 20 years, it's logical that your surge protection should be commensurate. The oft-heard complaint that "MOVs have a short lifetime" apply only to the dinky "Class II" SPDs popularized by spark gap manufacturers. A properly designed MOV SPD (rated according to the above chart) will withstand 20 years of large current lightning impulses. Beware of "SPDs" with a 1-year or 5-year warranty.
5) Field Proven & dependable. In 50 years of field applications it's been shown that MOV SPDs installed with the ratings stipulated in the above chart really do protect electronic equipment from actual lightning and provide no troubles to end-users.
What about the telecommunication lines entering a building. Aren't they also subject to to induced surges?
Yes, indeed they are. Because CIGRE's 2013 Technical Brochure 549 dealt mainly with power engineering, the effects of lightning on telephone lines, network lines, cable, etc. were not addressed. But a comprehensive lightning protection would have to include protection against overvoltages on the incoming telecommunications lines by means of appropriate data line protectors.
What about the grounding systems, lightning termination points, down conductors, bonding..........?
Any number of handbooks and standards address in great detail the subjects of grounding systems, lightning termination points, down conductors, and related issues. Most technical codes and standards agree on the basics of correct grounding. See extract from the excellent summary of grounding practices prepared by the National Lightning Safety Institute (www.lightningsafety.com). They pretty much all agree on the need of a single grounding system and that low impedance connections (bonding) must exist between all the different grounding and electrical systems at a site. This is a critical design point since it eliminates the potential voltage differentials that would appear were separate grounding systems allowed.
Ground resistance
Since we're on the subject of grounding we should briefly mention the subject of ground resistance values. When surge protection based on the 10/350 waveform fails, the most commonly heard excuse is "your 5-ohm or 10-ohm ground resistance is too high." An adequate grounding system is important, but effective surge protection can exist whether the ground resistance value is 5 ohms, 10 ohms, 25 ohms, or even higher. When a contractor is trying to sell you on exaggerated ground resistance values ("You need to reduce your ground resistance to under 1 ohm so your surge protection will work...") you are listening to someone who knows not of what he speaks.