Surge protection devices (SPDs)

Surge Protection Devices are used to protect equipment from the threats of fault current, which helps keep the valuable item working and railway lines running.
Holding Rail Product Box


Surge protection devices in some form are used on almost 100 percent of electrical applications. Often seen purely as a method of defence against a power surge from the incoming electricity lines, the currently applicable lightning protection standard BS EN IEC 62305:2006 requires measures to be installed and effectively deployed.

There is no "one size fits all" solution to protection against surges, or transient overvoltages, in the construction industry and the rail sector is no exception. As a result of specific locations requiring a defined application each individual package on a rail project is likely to have different needs. As rail systems are high dependency pieces of infrastructure and that operational downtime is tremendously costly it is vital that appropriate protection is applied so that service is not compromised, but under-resourcing the surge protection provision is likely to only lead to spiralling costs for remedial works at a later date.

Rail projects are at particular risk due to the hazards of lightning. This may seem unlikely based upon the presumption of ‘lightning strikes the tallest thing in an area’, which may commonly seem to be the case. This legacy mindset, which is all too frequently encountered in the construction industry, can simply result in millions of pounds worth of damage, lost revenue, operational downtime costs, and remedial work costs. The ways that lightning can impact on railway systems may include the following:

  • Direct strike to the structure: Lightning is essentially electrical charge seeking to discharge into the strata of the earth, and will channel through conductive structures or items that may facilitate this. Lightning creates the path to ground through a process of ionising air molecules. Rail systems, including all of their associated feeder poles and signals, etc. present an array of conductive opportunities. It is worth considering that lightning does not always behave in a predictable way; it all depends upon the ionisation path of air molecules.
  • Strike near to the structure: As lightning is essentially a significant discharge of electrical current it has the effect of creating an electromagnetic pulse, which will radiate out. If the pulse encounters any conductive item there is a potential that lightning current may be induced into, and through, that item. Consequently it will not take a direct strike for lightning current to damage items. When we consider that rail systems cover large expanses of ground and have a number of direct interfaces with other surrounding structures the potential exposure to the hazards of lightning increases. As lightning will always seek the path of least resistance this may lead to side-flashing, which is when lightning seemingly jumps from one structure or item to another, if there are items with different electrical potentials. This is a key issue and why equi-potential bonding is such an important process.
  • Direct strike to a service connected to the structure: Lightning can strike a range of objects on its path to ground and frequently this is incoming services. Aerial power lines and telecommunication lines which are connected to a structure can therefore provide a path inside for lightning current. As such all incoming lines should be assessed for the threat of lightning.
  • Strike near to a service connected to a structure: This potential route for lightning current is comparable to that detailed above. If lightning current strikes ground, or a nearby structure, then the electro-magnetic pulse can induct the current into the nearby service. This is especially true when conductive services are underground and lightning current is dissipating within the strata of the earth and psses near to the service.

On rail projects anything which enters a signal centre that carries a power supply should be considered for the application of surge protection devices. Knowing how and when to apply surge protection devices can be complex, but support is available. If you have any concerns about the deployment of surge protection of your rail project, or indeed any other sort of project in the construction sector, please do not hesitate to contact a member of the team.

Electrical safety products for railways



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