Surge protection in main power distribution systems

Surge protection in main power distribution systems
Surge protection in main power distribution systems
Published: 21 May 2020 - noon
By: Staff writer

If the connecting cables to surge protective devices are too long, reliable protection cannot be guaranteed. In order to also avoid liability risks here, correct installation is key in large main distribution systems. A protective device with integrated backup fuse is useful.

Surge protection in the incoming supply system has been mandatory at least since the publication of revised standards DIN VDE 0100-443 and DIN VDE 0100-534 in October 2016. For residential and commercial buildings with meter panel systems in the incoming supply, the surge protection is simply snapped onto the busbar system as a rule. In this case, there are no connecting cables, and a separate backup fuse is not required for the surge protection either. The upstream main fuse – in the building terminal box, for example - is almost always smaller than the maximum approved backup fuse of the installed SPD (surge protective device/overvoltage protective device).

Connecting surge protective devices

In larger main power distribution systems, it is not that simple. High nominal currents require significantly larger busbar systems and, in turn, larger distribution systems. There is room for surge protection, but the gaps and distances between system components is also greater. This often means that connecting cables to the SPD are much longer than the maximum of 0.5 m stipulated in the standards. This requirement is frequently underestimated, because it means that surge protection is not ensured.

Because the cable length of the surge protective device also impacts the effective voltage protection level in switching devices, DIN VDE 0100-534 specifies a maximum cable length of 0.5 m between active conductors and protective conductors. This half-meter applies in total from the pick-offs of the active conductors L1, L2, L3, and N from the busbar system to the connection bar for the protective conductor in total. If there is a backup fuse, the conductor route to the backup fuse must also be included in the calculation.

The reason for the specified short cable length is the voltage drop across the connecting cables in the event of a discharge event. On a 1 m long conductor routed in a straight line, a voltage drop of approx. 1kV is generated in the event of a high peak current of 10 kA.

Cable length impacts voltage protection level

This voltage drop must be added to the voltage protection level of the surge protective device. If an SPD with a voltage protection level of 1.5 kV is connected to a 1 m long cable, the effective voltage protection level in the electrical system is 2.5 kV. This applies to a high peak current of 10 kA – a value that equals approximately 50% of nominal discharge capacity per position in a type 2 SPD. In larger electrical systems, a type 1 SPD is normally used - this has a nominal discharge capacity of 25 kA per position and 100 kA in total.

At a high peak current of 25 kA, the voltage drop across a 1 m cable is already 2.5 kV – and at 100 kA, it is 10 kV. The voltage drop across the connecting cable must be added to the SPD’s voltage protection level. The total voltage quickly exceeds the voltage resistance of the devices to be protected, meaning that they can become damaged. Dangerous sparks and in the worst case, fire, are the result.

Installing surge protective devices in the right place

A surge protective device should always be installed as closely as possible to the system’s protective conductor in order that the connecting cables from the SPD to the protective conductor are as short as possible. The calculation is simple: For a type 1 SPD, a high peak current of 25 kA per position is to be expected for lightning protection level I. For four positions, the total current that flows to the protective conductor is therefore 100 kA. Therefore, the current to the protective conductor causes a voltage drop that is four times higher than it is on the connecting cables of the active conductors.

Nominal currents below 315 A are rare in larger main energy distribution systems. Therefore, the SPD requires a backup fuse. An external backup fuse for the surge protective device takes up room and cable length, and increases costs. This cable, which is part of the SPD’s connecting cable, additionally increases the voltage protection level.

The correct size of the fuse also plays a role. An NH00 fuse with 125 A, which, however, trips at lower high peak currents, which also flow through the upstream fuse, is conventional. To safely discharge the nominal discharge current of 25 kA per position without tripping it, a backup fuse rated at 315 A must be used. For this, at least an NH2 fuse is necessary. They are not only significantly larger than NH00 fuses, but more expensive as well.

Surge protective device with integrated backup fuse

One solution is an SPD with integrated fuse . This eliminates the need for a backup fuse, saving both space and costs. And because the surge protective device is not connected via the “detour” of a backup fuse, cable lengths can be shorter. Eliminating the need for a separate backup fuse opens up new options for installing the SPD. A surge protective device does not have to be operated or maintained – it is to ensure safety in the background. And that is exactly where it should be installed.

The FLT-SEC-H surge protective device from Phoenix Contact is a combination of a spark gap without line follow current and a fuse with a high surge current strength, and can be used without a separate backup fuse. With a short-circuit current rating of up to 100 kA, the protective device can even be installed in large power distribution systems upstream of the circuit breaker.

A surge protective device does not need to be operated or maintained. The FLT-SEC-H is also robust; the individual protective plugs rarely need to be replaced. However, during the regular dielectric test it is useful if they are pluggable.

Before a dielectric test, the protective plugs must be pulled out. If that is not possible, the connecting cables must be removed from the entire surge protective device. After the test, the cables are connected with a defined torque. This not only requires a lot of time but is also a potential source of error.

Convenient check via status indicator

The status of the FLT-SEC-H can be checked via the status indicator. Green means that the surge protective device and integrated fuse are OK and ready for use. This status can also be evaluated via remote indication contact; an SPD failure is immediately detected.

Another advantage is that the FLT-SEC-H also monitors the integrated backup fuse. For external backup fuses such as NH fuses, monitoring is not a matter of course. But when a backup fuse has tripped, even the best surge protective device is unable to provide protection.

In electrical systems with rigorous requirements on availability, the actual load on the protective plug can be checked. With the Checkmaster 2 test device, functional tests can easily be carried out on protective devices from Phoenix Contact. The protective plug is inserted into the test device and the electrical test provides a clear picture of the load on the protective plug so far. This means that the protective plugs can be replaced before they fail - a decisive criterion for systems that cannot be switched off outside of planned inspections.

Summary: Avoiding liability risks

The right selection, regular testing, and above all, the correct installation of surge protective devices are the keys to effective, safe protection. The pluggable FLT-SEC-H combined lightning current and surge arresters with integrated backup fuse offer multiple options for addressing these factors – therefore helping to avoid liability risks.

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Who is liable?

When damage occurs in an electrical system, people quickly start playing the blame game. Was planning responsible for the error? Or was it the design? Or is the operator the guilty party?

As soon as major damage and high costs are the issue, it becomes difficult to come to an agreement – and the case often lands in court. The effort and expense can quickly grow. It’s better to avoid liability risk completely. Planners, installers, and operators should all find out about selecting the right surge protection and installing it correctly.

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