Created by: Glen Zhu | Updated Date: Aug 27, 2022
LSP Guide to Surge Protective Devices
Selection, application and theory
Electronic systems now pervade almost every aspect of our lives, from the work environment, through filling the car and even shopping at the local supermarket. As a society, we are now heavily reliant on the continuous and efficient running of such systems. The use of computers, electronic process controls and telecommunications has ‘increased exponentially’ during the last two decades. Not only are there more systems in existence, the physical size of the electronics involved has reduced considerably. This reduction in size means less energy is required to damage components.
A Surge Protection Device (SPDs) is a component of the electrical installation protection system. This device is connected to the power supply in parallel with the loads (circuits) that it is intended to protect (see Fig. 1). It can also be used at all levels of the power supply network. This is the most commonly used and most practical type of overvoltage protection.
There are three types of SPD according to international standards:
The IET Wiring Regulations (BS 7671- Requirements for Electrical Installations) define the requirements for a safe electrical installation. Part of the criteria is adequate protection for both people and equipment from transient overvoltages of atmospheric origin transmitted via the supply distribution system and against switching overvoltages
A transient overvoltage or surge is a short duration increase in voltage measured between two or more conductors.
Transient overvoltages are by definition a very specific form of disturbance. It is therefore worth briefly outlining other forms of electrical disturbance in order to understand what transient overvoltages are not!
‘Outage’, ‘power cut’ and ‘blackout’ are all terms applied to total breaks in the supply lasting from several milliseconds to many hours.
‘Undervoltages’ or ‘brownouts’ are sustained reductions in the supply voltage, lasting anything over a few seconds.
‘Overvoltages’ are sustained increases in the supply voltage, lasting anything over a few seconds.
‘Sags’ or ‘dips’ are decreases in the supply voltage, lasting no more than a few seconds.
‘Swells’ (also called ‘surges’) are increases in the supply voltage, lasting no more than a few seconds.
Electrical noise or Radio Frequency Interference (RFI), is a continuous high frequency (5kHz or more) distortion of the normal sine wave.
Harmonics are a continuous distortion of the normal sine wave, at frequencies of up to 3kHz.
Nuclear ElectroMagnetic Pulse (NEMP), or ElectroMagnetic Pulse (EMP), are pulses of energy caused by nuclear explosions and intense solar activity.
ElectroStatic Discharge (ESD) is a different phenomenon.
ElectroMagnetic Interference (EMI) is a very broad term referring to system interference.
Transient overvoltages are generally caused by lightning and/or electrical switching events. Transient overvoltages can be generated by lightning, (through resistive, inductive or capacitive coupling) or by electrical switching events.
Lightning activity can cause transient overvoltages on both mains power supplies and data communication, signal or telephone lines.
Direct strikes to High Voltage (HV) power cables. Strikes to HV overhead power lines are quite common.
Resistive coupling is the most common cause of transient overvoltages and it will affect both underground and overhead lines.
Inductive coupling is a magnetic field transformer effect between lightning and cables.
Where long lines are well isolated from earth (e.g. via transformers or opto-isolators) they can be pulled up to high voltages by capacitance between them and charged thunder clouds.
Transient overvoltages caused by electrical switching events are very common and can be a source of considerable interference. Current flowing through a conductor creates a magnetic field in which energy is stored.
Nearly all electronic components and circuits suffer transient overvoltages damage in the same way. There are two main physical mechanisms at work, overheating and insulation failure – both are made much worse by the subsequent power follow-on.
Transient overvoltages, whether caused by lightning or by electrical switching, have similar effects: disruption, degradation, damage and downtime.
Although no physical damage is caused, the logic or analogue levels of the systems’ are upset.
This is somewhat more serious. Long term exposure to lower level transient overvoltages will, unknown to the user, degrade electronic components and circuitry reducing the equipment’s expected life and increasing the likelihood of failures.
Large transient overvoltages can cause damage to components, circuit boards and I/O cards. Severe transient overvoltages can physically manifest themselves through burnt-out circuit boards, however, ordinarily damage is less spectacular.
Unnecessary disruption, component degradation and damage all result in equipment and systems downtime.
The BS EN 62305 standard series specifically cover the protection against lightning to structures, their contents, persons and livestock.
Damage that can be caused by lightning is sub-divided into:
Each source of damage may result in one or more of three types of damage.
The following types of loss may result from damage due to lightning;
Our reliance as a society on electronic equipment has increased dramatically over the last two decades.
BS EN 62305 takes account of protection measures on metallic service lines (typically power, signal and telecom lines) using transient overvoltage or Surge Protection Devices (SPDs) against both direct lightning strikes as well as the more common indirect lightning strikes and switching transients.
Lightning currents as a result of direct lightning strikes are represented by the simulated 10/350µs waveform with a fast rise time and long decay that replicates the high energy content of direct lightning.
NOTE: Surge Protection Measures (SPM) were previously known as LEMP Protection Measures Systems (LPMS) in BS EN 62305.
Protection against LEMP is based on a concept of the Lightning Protection Zone (LPZ) that divides the structure in question into a number of zones according to the level of threat posed by the LEMP.
Given that the live cores of metallic electrical services such as mains power, data and telecom cables cannot be bonded directly to earth wherever a line penetrates each LPZ, a suitable SPD is therefore needed.
BS EN 62305 deals with the provision of SPDs to protect against both the effects of high-energy direct lightning strikes and indirect lightning strikes plus switching transients.
Designed to prevent dangerous sparking caused by flashover. Flashover is caused when the extremely high voltages associated with a direct lightning strike breaks down cable insulation.
Designed to protect electrical/electronic equipment from the secondary effects of indirect lightning and against switching transients.
It is fundamental to ensure the avoidance of dangerous sparking occurring within the structure to be protected.
Following a risk evaluation in accordance with BS EN 62305-2, the choice of suitable equipotential bonding SPDs is determined by a number of factors.
Partial lightning current (as defined by a 10/350µs waveform) can only enter a system through either a structure’s LPS or an overhead line as both are subject to a direct strike.
BS EN 62305-4 emphasises the use of coordinated SPDs for the protection of equipment within their environment.
The withstand voltage is the maximum value of surge voltage which does not cause permanent damage through breakdown or sparkover of insulation. This is often referred to as the dielectric withstand.
Correct installation of SPDs is vital. Not just for the obvious reasons of electrical safety but also because poor installation techniques can significantly reduce the effectiveness of SPDs.
The voltage drop on the connecting leads UL of an installed SPD adds to the SPD’s protection level UP as seen by the equipment. This is particularly the case for SPDs installed in parallel (shunt) on power installations.
If the distance between a parallel installed SPD and the equipment to be protected is too large, oscillations could lead to a voltage at the equipment terminals which is up to double the protection level of the SPD, UP.
Composite cables consist of more than one core. ‘Modes’ refers to the combinations of conductors between which surges occur and can be measured.
Protecting equipment from the risk of permanent failures or damage due to surges considers the withstand voltage UW as defined by IEC 60664-1 (see Table 44.3 as seen in Section 443 of BS 7671).
“Standard” SPDs may offer protection levels below the withstand level of the equipment or system they protect.
For the SPM designer there are considerations for the location of SPDs as detailed in Annex D of BS EN 62305-4.
BS 7671 Section 534 requires installed SPDs to be protected against short-circuit through the use of over current protective devices (OCPDs). Reputable manufacturers of SPDs provide clear guidance for the selection of the correct ratings of OCPDs in their SPD installation instructions.
The OCPD and SPD should be co-ordinated to ensure correct operation. It is important that discrimination between the SPD OCPD and upstream OCPDs is achieved in all installations.
Where the power distribution system incorporates RCDs transient activity could cause RCDs to operate and hence loss of supply. SPDs should wherever possible be installed upstream of RCDs to prevent unwanted tripping caused by transient overvoltages.
Where installed at the main intake switch panel or a distribution board the Surge Protection Device (SPD) should have a separate overcurrent protection device (OCPD), depending on the maximum prospective fault current at the point of installation.
The SPD connections should be as short as possible in order to reduce the value of the voltage protection level (installed Up) on the terminals of the protected equipment.
Where SPDs are installed a durable notice should be fixed adjacent to the distribution board(s) and equipment complying with the requirements of BS 7671.
Insulation Resistance Testing
BS 7671 Part 6 details insulation resistance testing with consideration of SPDs fitted within the installation.
When carrying out a periodic inspection and test of an existing installation it is important prior to applying any insulation resistance tests, to establish if the installation has any SPDs installed.
The object of the inspection is to verify the following:
LSP Guide to Surge Protective Devices
Selection, application and theory