Surge protection is increasingly being utilised to protect power networks from the threat of damage due to lightning strikes. Lightning damage can be catastrophic, but it could also be undetected over time, as the insulation of critical components can be gradually degraded by low level strikes before ultimately failing.

Protecting power circuits can be an obvious choice in many applications, however there are other parts of a plant protection concept that should not be ignored. To safeguard a plant and its operations, a multi-layered approach addressing various system vulnerabilities should be considered.

Consequences of surges

Modern industrial environments can rely on large scale integrated electronics, sophisticated sensors, and control units connected by copper cabling. Often these systems are located across large indoor /outdoor sites. The equipment used in these systems are costly and can be susceptible to electrical disturbances. Apart from the equipment cost, the economic impact of the process downtime to repair or replace damaged components along with the time to restart the plant often surpasses the cost of the failed item.

The surge related effects that can compromise a process control system can be caused by:

• earth potential rise (EPR)
• magnetic field coupling
• electric field coupling

EPR is considered the most damaging effect for process control systems with remote sensors around a plant, whereas electric and magnetic fields often influence cabling systems within a structure. The result of these surge related effects can be large voltage differences that can exceed the normal equipment ratings and damage the sensors and controls of the process.

There are two modes where transient voltages can affect equipment:

• Common mode, where a transient is present between the signal lines and ground (or an earthed component)
• Transverse mode, also known as differential or normal mode, where a transient is present between individual lines or cables

A surge protective device can provide appropriate protection against both these effects, relevant to the type of signal to be protected.

Recommendations of the standards

The Australian Lightning Protection standard, AS 1768:2021, defines Lightning Protection Zones (LPZ) throughout a structure. The standard recommends protection be placed where a conductive cable crosses from one zone to the next. At this junction of zones, also known as the point of entry, the standard refers to primary protection being required. In cases where a Lightning Protection System (LPS) is installed, primary protection is considered mandatory.

For example, the outside of a structure is LPZ0, the zone with the highest level of exposure to the effects of lightning. This zone is also split into LPZ0_A, where there is a threat of direct strike, and LPZ0_B, where there is protection from a direct strike but there is exposure to the full electromagnetic effects of a strike. Entering the structure the zone changes from LPZ0 to LPZ1, where there is a reduced threat of exposure.

Inside the structure the zone can change from LPZ1 to LPZ2, or LPZ2 to LPZ3 indicating further reductions to the threat. At these junctions of zones, the standard requires secondary protection to be installed in accordance with the risk assessment described in the standard. Primary protection at the point of entry is designed to prevent dangerous discharges by diverting much of the energy of the surge and prevent damage to the secondary protection. The secondary protection ensures the sensitive equipment is protected.

The standard recommends secondary protection in cases where:

1. the value of Ng (flashes /km²/year) is greater than 2 and conductive services are overhead
2. the consequences of surge damage could affect human safety
3. the consequences of surge damage could affect public service utilities
4. economic consequences of surge damage are intolerable

The recommendations for signal/data SPDs in the standard are shown in the following table:

The I_max rating is a maximum discharge rating based on a 8/20 µs waveform associated with a transient or surge. The I_imp rating is an impulse discharge current rating based on a 10/350 µs waveform, usually associated with lightning.

The standard further identifies secondary protection requirements when:

1. sensitive equipment is present
2. the distance between an SPD located at the entrance and the equipment to be protected is too long, typically > 10m
3. there is internal equipment generating switching surges, or other internal interference sources, inside the structure

Field sensors and other devices are often in exposed environments and their failure can be a major contributor to process downtime, so a well-designed protection system should consider protection locally at these sensors as well as at the process control equipment cabinet.

There is a wide variety protocols used in industry, and there are SPDs available from Dehn and Novaris designed to suit the protection requirements of each application. When selecting a product for process control surge protection consideration needs to be given to:

• Voltage (e.g. 0 – 10 V)
• Current (e.g. 0 – 20 mA, 4 – 20 mA)
• Type of signal transmission (balanced, unbalanced)
• Frequency (DC, LF, HF)
• Type of signal (analogue, digital)

To assist in choosing the correct protective device, selection tables are available.

IPD offers a full range of solutions from world class surge protection specialists Dehn and Novaris, ensuring the best protection and advice for every application.