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Lightning suppressors, anti-surge devices and PV Solar Panels - tutorial and FAQ

  • 21 November 2020
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Lightning suppressors, anti-surge devices and PV Solar Panels - tutorial and FAQ
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Lightning suppressors: :zap:

A lightning suppressor is similar to a mains-connected anti-surge device. However, the magnitude of the pulses it needs to quench are significantly greater.

These suppressors are rather like an insurance policy. It isn’t essential to have them, and each person will have differing ideas about the level of risk they are prepared to take. As I live on high ground with a clear view of the prevailing SW wind from the Atlantic, it’s a subject in which I have a keen interest!

 

A short tutorial:

Lightning is caused by electrostatic charge building up within clouds. The higher the cloud, the greater is the potential difference (voltage) between its base and its crest. The ground is positively charged in comparison with the clouds above us. For this reason, lightning tracks are normally measured as volts-per-metre (V/m). The higher the cloud, the greater is the voltage to earth.

 

Globally there are about 100 million lightning discharges each year, of which 25 million (25%) are ground-strikes. They are not evenly spread. High ground in Rwanda and the Black Forest will typically have several strikes each day. For this reason I like suppressors made by the German firm, Phoenix Contact. They are not only well engineered, but get lots of testing!

If you put metal objects on your roof, such as solar panels and wind turbines, they will attract lightning discharges because there’s a route to ground with lower resistance through metal cables.

 

A pair of lightning suppressors are connected to the +ve and -ve wires from the solar panel before they enter any other electronic equipment.

The other side of each suppressor connects to a ground stake via a 10mm² earth wire.

Do not attach this earth wire back to the earth in your consumer unit. To do so will re-route some of the lightning surge back into your house and to all your neighbours on the same phase!

 

What lightning “looks like”:

A cloud to ground strike is not a single event, but usually a series of 3 - 5 discharges with a few millisecond intervals between them. This is why we perceive lightning as flickering.

The following diagram is derived from work by V.A. Rakov ‡  and zooms in on just one of those discharges in the sequence:

 

The main Leader is preceded by a number of initial pulses, caused by the high voltage breaking down the air into ionised particles.

This plasma pathway channels through the Leader of the main strike which is rapidly followed by the Return Stroke as the ground re-establishes the equilibrium. Over the next 1-3mS there are a series of Pulse Peaks before the next Leader and Return Stroke pair in the sequence.

 

A lightning suppressor needs to act very fast.

Electromagnetic radiation in copper wire travels at two-thirds the speed of light. In the 5μS between the Leader and the Return Stroke, the electron wave-front will have traveled a kilometre along a cable!

The suppressor must also self-heal rapidly. There’s no point in it quenching the initial Leader (A) if it can’t be ready to fire again for the succession of Pulse Peaks (B) which follow.

It is this which underlies the significant difference in price between Surge Protection Devices. It is almost impossible for a consumer to know the resilience of an SPD. It’s important to check the standards it has been tested against rather than the headline specification.

 

Although a lightning discharge is of short duration, the current from a direct strike flows at up to 40,000 Amps. With a typical Potential Difference in the range of tens of megavolts, no amount of insulation around a wire is going to put up much resistance (pun intended!).

Wires connecting to a Suppressor need to be thick and kept short, especially the earth-wire to the ground-stake. Every screw terminal must be extremely tight to ensure that it really is the route through the Suppressor which offers the lowest resistance path to earth.

 

A lightning Suppressor will be marked with its maximum current and the breakdown voltage at which it “breaks down” to carry the current to ground. Most protection devices intended for solar panels are rated at 40kA and 1000v DC.

DC Surge Protection Devices from Phoenix Contact (Germany) and EARU (China)

 

Sometimes, the Suppressors are built into the input of the PV Inverter. But, if not, then this should be preceded by a DC Connection/Combiner Box where the Suppressors can be fitted very close to the fuses and circuit-breakers for each PV string.

 

PV Panel; string combiner box under construction

Make sure that Suppressors are connected to the solar-cable before the circuit breakers (shown here with coloured operating levers).

A DC circuit-breaker contains an arc-quenching “ladder”, which is needed to prevent the contacts arcing and welding themselves together when the circuit is switched off.

comparison of two DC circuit breakers from Chinese manufacturers

If the lightning discharge hits the circuit-breaker first, then the internal magnet will pull the pulse into the arc-quenching ladder and destroy the breaker.

Should that occur, there will be no route through to the lightning suppressor for the following Leader and Pulse-peaks. Consequently they will arc across wherever they can within the enclosure, probably resulting in a fire. :scream:

 

   V.A. Rakov and M.A. Uman, 2003: Lightning: Physics and Effects, Cambridge University Press


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FAQ:

 

Q: I have an inverter fed from my PV Solar Panels. It states that it contains surge suppression. Does this mean my system is protected from lightning?

A: Not necessarily. Surge suppression could just as well be referring to spike-quenching on the 230v AC output to the mains. See next section, below.

If a device contains lightning suppression, then you would expect to see a separate earth terminal. The installation instructions would require this to be connected to an external earth-stake with a heavy-duty cable (minimum 10mm²).

Without such an earth connection, it’s unlikely that your Inverter would survive a lightning discharge through your PV Panels.

 

Q: Why has my installer fitted a large rotary switch next to my PV Inverter?

A: The G98, G99 & G100 specifications for installation of equipment which is connected to the Grid require that there is a dual-pole, lockable isolator to disconnect the inverter or Storage Battery from the mains.

It is usual that this is a rotary switch because it has slots through which an electrician can install a padlock. This ensures that no one else can switch the system back on whilst he’s working elsewhere in the house.

 

Q: What if I want to isolate my PV Solar Panels from the inverter?

A: The cables from your Solar Panels may also have a DC rotary-isolator. This could be coloured black rather than yellow/red.

Most DC rotary switches do not have an arc-quenching system within them. For that reason, they are usually connected with a DC Circuit-Breaker in front of them, and commonly a pair of fuse-holders too:

To disconnect the PV Panels from the inverter, first move the Circuit Breaker lever to off, then turn the rotary isolator switch.

Safety: Remember that an array of PV Panels is never really off, even at night. If you open the fuse-holders, always assume that they’ve been wired the wrong way around. Don’t touch both fuses at once because you risk putting several hundred volts through your hand.

It’s good practice that the lightning suppressors are connected to the input side of the Circuit Breaker. This means that they remain connected to the solar panels when the Breaker is turned off.

Userlevel 7

Boom! Another great guide for the world wide web and search engine crawlers to pick up on! 

 

One of your more dramatic guides, @Transparent :zap:

 

 

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Mains electricity - anti-surge suppressors.

Although they operate in a similar way to a lightning protection device, a surge suppressor on the 240v mains has to cope with a wider variety of different sorts of transient voltages.

In the worst case you might want a mains suppressor to quench a lightning discharge which is being propagated through the 240v cabling. This most commonly occurs in rural areas where there are long runs of overhead wires strung between poles across the countryside. This coarse level of anti-surge protection is referred to as Type-1.

At the minor end of the scale any suppressor should be capable of nullifying transient spikes caused by motors switching off - typically vacuum cleaners or poor-quality electric drills. When an electric motor stops, it has to release the electro-magnetic energy built up in its coils. This creates a spike called the back-emf (electromotive force), which travels back down the wires in your house.

Type-2 suppressors will quench these high-level spikes. They require a substantial earth connection through which to send the excess energy.

Type-3 suppressors are more like filters. They allow the low-frequency mains (50Hz) to pass through, but present a greater impedance to high-frequency noise and switching transients. They are found in some 4-way extension leads for example.

 

mains-borne transients

Transient spikes between 500-1000v can affect electronics found in household devices such as TVs, computers, WiFi routers and chargers.

The voltage will momentarily exceed the specified value of components such as capacitors, causing them to become leaky. Within power-supply units, these capacitors are used to ensure that the low-voltage outputs are smooth, without which, radio-frequency devices like radio and TVs can become noisy.

Surges above 1kV can cause more lasting damage, particularly intermittent faults that are hard to trace.

 

The best place to fit a pair of Type-2 mains surge suppressors is in the Consumer Unit, close to the MAIN SWITCH. This guards against incoming surges from the grid supply and also quenches transients on one circuit before they get passed to another.

 

Suppression devices within a Consumer Unit (aka Distribution Board) must be installed by a qualified electrician. It is illegal for unqualified members of the public to work on the mains wiring in a house.

There is wide range of surge suppression units available, with prices ranging from $8 directly from a Chinese manufacturer, to £150 for a pair manufactured by a company in Europe.

Mains surge protection from Phoenix Contact, British General and Wylex (Electrium)

There is obviously no point in having an anti-surge device which doesn’t operate when it’s needed. If the specifications appear incomplete, aren’t backed up with test certification, or design standards references contain spelling mistakes(!) then it’s probably not worthwhile parting with your money.

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