HomeLightning & Surges ProtectionElectronic Systems Protection › Furse Electronic Systems Protection Guide

Furse Electronic Systems Protection Guide

What Transients Are & Why You Need Protection

Transient overvoltages are short duration, high magnitude voltages peaks with fast rising edges, also known as surges. Often described as a 'spike', transient voltages can reach up to 6000V on a low-voltage consumer network, with no more than millisecond duration.

Lightning strikes are the most common source of extreme transient overvoltages where total outage of an unprotected system can occur with damage to cabling insulation through flashover potentially resulting in loss of life through fire and electric shock.

However, electrical and electronic equipment is also continually stressed by hundreds of transients that occur every day on the power supply network through switching operations of inductive loads such as air-conditioning units, lift motors and transformers. Switching transients may also occur as a result of interrupting short-circuit currents (such as fuses blowing).

Although switching transients are of a lower magnitude than lightning transients, they occur more frequently and equipment failures unexpectedly occur often after a time delay; degradation of electronic components within the equipment is accelerated due to the continual stress caused by these switching transients.

Transient overvoltages, whether caused by lightning or by electrical switching, have similar effects: distruption (e.g. data loss, RCD tripping), degredation (reduced equipment lifespan), damage (outright equipment failure, particularly concerning for essential services such as fire and security alarm systems) and downtime - the biggest cost to any business such as lost productivity and product spoilage, staff overtime, delays to customers and sales lost to competitors.

How To Get Effective Electronic Systems Protection

In order to provide effective protection, a transient overvoltage protector/SPD must:

  Be compatible with the system it is protecting
  Survive repeated transients
  Have a low `let-through' voltage, for all combinations of conductors (enhanced SPDs to BS EN 62305)
  Not leave the user unprotected, at the end of its life, and be properly installed


The protector must not interfere with the system's normal operation:

  Mains power supply SPDs should not disrupt the normal power supply such as creating follow current that could blow supply fuses, or cause high leakage currents to earth
  SPDs for data communication, signal and telephone lines should not impair or restrict the systems' data or signal transmission


It is vital that the protector is capable of surviving the worst case transients expected at its installation point/LPZ boundary. More importantly, since lightning is a multiple event, the protector must be able to withstand repeated transients.

The highest surge currents occur at the service entrance (boundary LPZ 0A to LPZ 1). For buildings with a structural LPS, the lightning current SPD could be subject to as high as 25kA 10/350?s surge currents per mode on a 3-phase TNS mains system (up to 2.5kA 10/350?s per mode on a signal or telecom line) for a worst-case lightning strike of 200,000A.

However, this 200kA level of lightning current itself is extremely rare (approx. 1% probability of occurring) and the peak current the SPD would be subject to further assumes that a structure is only fed with one metallic service. Almost all structures have several metallic services connected to them such as gas, water mains, data & telecoms. Each service shares a portion of the lightning current when the protected building receives a strike, greatly reducing the overall current seen by any single service, and as such any SPD fitted to the electric service lines.

Transient overvoltages caused by the secondary effects of lightning are considerably more common (lightning flash near a connected service up to 1km away from the structure) and therefore are unlikely to have currents exceeding 10kA 8/20?s.

Let-through voltage

The larger the transient overvoltage, the greater the risk of flashover, equipment interference, physical damage and hence system downtime. Therefore, the transient overvoltage let through the protector (also known as the protection level Up of the SPD) should be as low as possible and certainly lower than the level at which flashover, interference or component degradation may occur.

Transient overvoltages can exist between any pair of conductors:

  Phase to neutral, phase to earth and neutral to earth on mains power supplies
  Line to line and line(s) to earth on data communication, signal and telephone lines

Thus, a good protector (enhanced SPDs to BS EN 62305) must have a low let-through voltage between every pair of conductors.

Enhanced performance SPDs - SPD*

BS EN 62305-2 details the application of improved performance SPDs to further lower the risk from damage. The lower the sparkover voltage, the lower the chance of flashover causing insulation breakdown, electric shock and fire.

SPDs that offer lower voltage protection levels further reduce the risks of injury to living beings, physical damage as well as failure and malfunction of internal systems. All Furse ESP protectors offer such superior protection and are termed as enhanced performance SPDs (SPD*) in line with BS EN 62305.

Enhanced SPDs can also satisfy more than one test class/category by handling both high-energy partial lightning currents of 10/350?s waveshape whilst offering very low let-through voltages.

Such enhanced SPDs may be suitable for changing a lightning protection zone from LPZ 0A right through to LPZ 3 at a single boundary or installation point. As such they provide both technical and economic advantages over standard SPDs.

End of life

When an SPD comes to the end of its working life it should not leave equipment unprotected. Thus in-line protectors should take the line out of commission, preventing subsequent transients from damaging equipment. SPDs for data communication, signal and telephone lines and protectors for low current mains power supplies are usually in-line devices.

Where SPDs are installed at mains power distribution boards it is usually unacceptable for these to suddenly fail, cutting the power supply. Consequently, to prevent equipment being left unprotected, the SPD should have a clear pre end-of-life warning, which allows plenty of time for it to be replaced.


The performance of SPDs is heavily dependent upon their correct installation. Thus, it is vital that SPDs are supplied with clear installation instructions. The following is intended to supplement the detailed guidance given with each product in order to give a general overview of installation. This should not be viewed as a substitute for the Installation Instructions supplied with the SPD. Copies of these are available separately on request.

Installing parallel connected SPDs for mains power supplies:

  SPDs should be installed very close to the power supply to be protected, either within the distribution panel or directly alongside of it (in an enclosure to the required IP rating)
  Connections between the SPD and phase(s), neutral and earth of the supply should be kept very short (ideally 25cm or less, but no more than 50cm)
  SPD performance is further enhanced by tightly binding connecting leads together (simply using cable ties or similar), over their entire length
  For safety and convenient means of isolation, the phase/live connecting leads should be suitably fused using HRC fuses or switchfuse, MCB or MCCB

Installing in-line SPDs for data, signal, telephone or power:

  SPDs are usually installed between where cabling enters or leaves buildings and the equipment being protected (or actually within its control panel)
  The installation position should be close to the system's earth star point (usually the mains power earth) to enable a short and direct connection to earth
  In-line, or series, connected SPDs generally have connections marked line and clean. The line end of the SPD should be connected to the incoming or "dirty" line (from where the transient is expected). The clean end of the SPD should be connected to the line or cable feeding the equipment
  Cables connected to the SPD's clean end should never be routed next to dirty line cables or the SPD's earth bond
  Unless ready boxed, SPDs should be installed within an existing cabinet/cubicle or in an 144 enclosure to the required IP rating

How to apply protection

Transient overvoltages are conducted into the sensitive circuitry of electronic equipment on power and data communication, signal and telephone lines. Protection is recommended for:

  All cables which enter or leave the building (except fibre optic)
  The power supply local to important equipment
  Electronic equipment outside the main building(s)

Protect incoming and outgoing electrical services

Lightning strikes between clouds or to ground (and objects upon it) can cause transient overvoltages to be coupled onto electrical cables, and hence into the sensitive electronic equipment connected to them. To protect the electronic equipment inside a building, all cables that enter or leave the building must be protected. Cables leaving the building can also provide a route back into the building for transients.

For each building protect incoming/outgoing:

  Mains power supplies (including UPS supplies)
  Data communication and local area network cables
  Signal, control, instrumentation and alarm lines
  CCTV, satellite, TV and antenna cables
  Telephone and telemetry lines

Protect the power supply locally to important equipment

In addition to installing protection on the mains power supply as it enters/leaves the building, protection should also be installed locally to important equipment. Protection at the main LV (low voltage) incomer(s) is necessary to prevent large transients from entering the building's power distribution system, where they could have farreaching effects.

However, where the cable run to equipment exceeds approximately 20 metres, transient overvoltages may appear on the mains after the protector at the main LV incomer.

These transients can result from:

  The electrical switching of large inductive loads within the building
  A lightning strike to the building - as lightning currents flow through down conductors transient overvoltages can be induced on to nearby power cables
  The natural inductance and capacitance of long cable runs, `amplifying' the voltage `let-through' the protector at the main LV incomer
Additionally, local protection guards against the possibility of a supply which enters/leaves the building being overlooked and left unprotected.

Protect data lines locally

Generally, the biggest risk to data, signal, telecom and network wiring is associated with cables that enter and leave the building. These should always be protected. However, data cables within a building can additionally have transients induced on to them when loops between data and power cables "pick up" voltages from the magnetic field caused by a lightning strike.

As part of the overall LEMP Protection Measures System (LPMS), BS EN 62305 advocates the use of metal in the structure, and a Faraday cage lightning protection scheme to help exclude magnetic fields. Cable management practices eliminate loops by routeing data and power cables along the same general path.

In these cases, the need for local data line protection is minimal. However, where these steps are not possible, data line protection, local to the equipment requiring protection, should be considered.

Protect electronic equipment outside the building

Onsite or field-based electronic equipment with mains power, data communication, video, signal or telephone line inputs will need to be protected against transient overvoltages. It may be helpful to think of each equipment cabinet or cubicle as a separate building with incoming/outgoing cables to be protected.

Complementary techniques

As well as the use of transient overvoltage protectors, BS EN 62305 outlines additional protection techniques, which can be used to help reduce the transient threat as part of the overall LPMS. These are described further in the Furse document "A Guide to BS EN 62305:2006 Protection Against Lightning." Where these can be used, principally on new build or refurbishment projects, they need to be supported by the use of SPDs.

Please note - Product designs and specifications are subject to change without notice. The user is responsible for determining the suitability of this product.

Back To Last Page


Earthing & Lightning Protection

Electronic Systems Protection (ESP Range)


Ty-Met Stainless Steel Cable Ties

Archive Information

Archive Furse Information

Want A Quotation?

Please fill out the following fields and we will e-mail you a quotation shortly.