PARAFOUDER : Surge arrester performances

You can buy the "best" lightning arrester in the world, if you have not dealt with the points seen above:

Type I, Type II and Type III arresters

The lightning arresters of Types I are intended for sites equipped with lightning conductors and where the ground interconnections are not made: This does not concern us, we are interested in lightning arresters intended for the protection of electronic equipment.

The arresters of Type II and Type III are the arresters you use to protect your equipment.

Surge arrester dealers will insist on one or other of the characteristics of their surge arresters depending on what they want to emphasize:

"My lightning arrester is the best because it's the fastest."

"My surge protector is the best because it has the biggest flow capacity"

"My lightning arrester is the best because it's the cheapest."

"My arrester is the best because it has the lowest residual voltage."

In short, the best thing to do is to understand how it works and thus avoid being taken for a ride.

Six characteristics of surge arresters that are technically referenced:

-Common Mode / Differential Mode

-Residual surge Up,

-Flow capacity Imax,

-Nominal discharge current In,

-Static ignition voltage Uc,

And one that unfortunately imposes limits: -The price.

Common Mode / Differential Mode

Some lightning arresters incorporate common-mode only protection, others common-mode AND differential-mode protection. The first surge arresters protect against overvoltages between 1 wire and earth, the second surge arresters protect against overvoltages between wires.

Residual overvoltage "Up" of an arrester

Also known as the protection level for surge arresters on the power grid, this characteristic is one of the essential values of a surge arrester.

Up expresses, in kV, the voltage level that the arrester allows to pass in the direction of the product to be protected. It is defined by the manufacturer following tests carried out on the arrester underspecified conditions. .

But beware...

-If you want to compare the Up of two surge protectors, the In and Imax of these surge protectors must be identical,

-The "ideal" conditions used for surge arrester tests are only very rarely found on site, so it is imperative to take the greatest possible care in the installation of surge arresters in order to achieve the ideal conditions that allow the results announced by the surge arrester manufacturers to be obtained.

-Up expresses, in kV, the voltage level that the arrester allows to pass in the direction of the product to be protected against lightning. This voltage depends on :

- The current that runs through the lightning arrester,

- The flow capacity In of the arrester,

- The value of the static surge arrester's ignition voltage,

- The reaction speed of the lightning arrester.

- Ground impedance (Wiring in series or parallel of the lightning arrester?)

In all cases and all other things being equal, the lower the value indicated, the "better" the arrester is.

The orders of magnitude of the most common residual voltages for surge arresters are :

Rated voltage Residual voltage

400 V : 2500V

230 V : 1800V 1500V 1200V

Telephone: 200V

Signal lines: 10 to 150V

Rated discharge current "In" of an arrester

Product Catalogue

In can be assimilated to the lightning shocks that can "usually" flow from the arrester.

In reality more interesting than Imax, this characteristic is often neglected because, in order of magnitude, In ~ ½ or 1/3 Imax

Flow capacity Imax

Imax can be compared to the largest lightning shock that the arrester can deliver without being damaged.

This magnitude, expressed in kA, must be determined in accordance with the lightning exposure level of the installation site. see also

This level of exposure to lightning depends, as seen above, on various cumulative factors:

- Local lightning density.

- The length and location of the HTA, BT, RTC, LS lines for connection to the sensors.

- The topography of the site.

- Line and site environment.

- The quality of the installation's earth network. The higher this quality (low impedance), the higher the current that will pass through the lightning protection system.

By taking these factors into account, it is possible to determine the flow characteristics of the surge arresters to be selected. The higher the risk exposure, the greater the flow capacity of the arrester will need to be.

We have seen that the average and maximum values of a direct lightning strike have been defined at 40 kA on average and 100 kA for the maxima. Lightning arresters are never subjected to direct shocks since they are connected to the lines. Consequently they never have to drain the currents of direct shocks. It can therefore be considered that 40 kA of flow capacity for a surge arrester allows in all cases to cope with the most unlikely situations.

In urban areas a low value, 2.5 kA may be sufficient.

In very exposed isolated areas (mountainous areas for example) 40 kA may be considered necessary.

In the world of water:

Flow capacities of 8 to 10kA are generally selected for "low" exposed sites, i.e. urban sites (urban substations, urban reservoirs, etc.).

Flow capacities of 15 to 20kA are generally selected for the most exposed sites, i.e. reservoirs on towers, isolated pumping stations, equipment in rural areas, etc.

Static ignition voltage of the arresters: "Uc

Product Catalogue

This is the voltage which, in direct current, causes the lightning arrester to be triggered.

This value is specified for a current of 1 mA or less.

It is of course imperative that the Uc of the lightning protection is higher than the peak operating voltage.

Two cases are possible: Uc close to the peak operating voltage and Uc far from the peak operating voltage.

The closer the Uc of the arrester is to the peak operating voltage, the more effective the lightning protection is, but it is then solicited for each temporary low amplitude overvoltage and its life is limited.

The other possibility, a lightning arrester Uc away from the peak operating voltage, leads to the opposite situation: low efficiency and long service life. But, a remote Uc is not a concern as long as you have paid attention to select a surge arrester whose Up is lower than the immunity level of the equipment you want to protect.

It is possible and desirable to check the evolution of this ignition voltage with a voltage ramp generator to determine the real exposure of the installation to the risk of lightning. This check will also make it possible to determine whether the arrester is close to its end of life and to proceed with replacement of the arrester before it causes an incident. The correct level of this ignition voltage must in fact be defined by the lightning protection manufacturer. The result of this choice will result in a higher or lower residual voltage and a longer or shorter service life of the arrester.

For power networks in IT regime, the user of lightning protection will check that the static ignition voltage of the lightning arresters is higher than that of the insulator of the installation. If this is not the case, an insulation fault on the installation will cause the lightning protection devices to trip so that they flow the fault current to earth instead of the insulator.

In summary,

you just have to choose the lightning arrester:

-Common and differential mode,

-Wiring in series,

-Imax as big as possible,

-In as big as possible,

-Up as low as you can,

-Uc as big as possible,

But then there's the price.

The price

The price of a lightning arrester is directly related to its flow capacity. In the choice of a surge arrester, the price is taken into account.

In order to estimate the usefulness of protecting a system and to assess the price to be paid for it, only the price of the equipment to be protected is often taken into account.

This overly restrictive approach does not take into account the cost of using lightning protection, which is the only relevant economic element.

For defined characteristics this cost is determined according to the following elements:

1) Surge arrester lifetime Product catalogue

The service life of the arrester is proportional to the difference between Uc and the peak operating voltage. It also varies according to the energies flowing through the arrester. However, 2 surge arresters with the same static ignition voltage but different flow capacities can be priced in a ratio of 1 to 4. This difference will leave no buyer indifferent. However, the service life of these arresters can vary in a ratio of 1 to 1,000. Taking into account the cost of use will leave no manager indifferent.

As an example, a zinc oxide varistor with high flow capacity can withstand approximately :

1 lightning strike at 20 kA

10 lightning strikes at 10 kA

100 lightning strikes at 5 kA

1,000 lightning strikes at 2.5 kA

In urban areas a low-flow arrester is sufficient, it will cost 4 times less to purchase than if you choose a high-flow arrester. But to obtain the same lifetime, you will have to buy 1000 times more.

As always, the manager will have to choose between capital and operating costs.

2) The cost of replacing a lightning arrester Product catalogue

For completeness, in the previous example, the time needed for the 1000 surge arrester replacements and the related trips must also be taken into account.

All in all,

we therefore retain the lightning arrester presenting :

1) Common mode and differential mode protection,

2) Wiring in series,

3) An In au adapted to the site, i.e. :

> 2.5 kA in urban areas

> 5 kA in rural areas

> 10 kA in isolated mountainous areas

4) An Up (residual voltage) adapted to the equipment to be protected,

5) An Imax adapted to the site:

> 5 kA in urban areas

> 10 kA in rural areas

> 20 kA in isolated mountainous areas

6) A high priming threshold.

End of life of surge arresters

A) On the LV network Product Catalogue

Surge arresters, on the LV network, are always placed downstream of the general protection device.

This is usually a circuit breaker which can be :

- non-differential,

- instant differential,

- S-type differential,

- Time-delayed differential.

To limit the tripping of the differential devices, it is recommended to choose either a time-delayed differential or a type S differential (45 ms delayed operation). These circuit-breakers provide protection against tripping due to lightning shocks of around 5 kA carried by the power distribution network lines.

They must not perform the thermal protection function of the arresters. This function must be provided by automatic disconnection either internal or external to the arrester. This arrester disconnection system interrupts, by opening the circuit, the thermal runaway of the arrester caused by the heating of the varistor subjected to the weak flow of a permanent current. When this thermal protection is internal to the arrester and placed in parallel on the installation, a visual sign must indicate that the protection of the installation is no longer ensured by the arrester.

When this status indicator or "end of life indicator" is activated, it means that the arrester is out of service but when the arrester is out of service it is rare that the indicator is activated (it will only be activated if the "end of life" of the lightning protection is due to a thermal runaway of the arrester)

B) On other types of lines. Product Catalogue

Like the lightning protection of power lines, lightning arresters placed on telephone or signal lines die in a short circuit. Always, or almost always, installed in series on the line, the short-circuit arrester alters the signals or information present on the line. The end-of-life information of the arrester is then unequivocal.

The standards obviously do not require thermal disconnection on these arresters because, as they are not energy lines, there is no thermal runaway: thesignage would then be purely decorative.

Chapter 1: LIGHTNING PROTECTION: Lightning Reminders

Chapter 2: PROTECTION AGAINST DIRECT SHOCKS

Chapter 3 PROTECTION AGAINST INDIRECT LIGHTNING STRIKES

Chapter 4 : PROTECTION AGAINST INDIRECT FOUNDATIONS (continued)

Chapter 5: PROTECTION PERFORMANCES OF FOUNDATIONS

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