SPD's and Protection against Transient Voltage Surges
To-day, the demand and dependency on electronic equipment and systems is growing exponentially in every section of the society, particularly among the urban middle class. Electronic equipment such as personal computers, printers, modems, DVD players, home theaters, telephones, micro-ovens and satellite dish are common in many residential buildings and business establishments. The functions of these systems are controlled by in-built microprocessors and micro devices. The flip side of these electronic devices and systems are their potential vulnerability to transient over voltages, called “surges”. Surges are unwanted electrical disturbances that may couple onto the power, communication and signal lines causing damage to electronic equipment. The occurrence of common failures such as corruption of data, equipment shutdown, and loss of power are due to surges. Application of suitable Surge Protection Devices (SPDs) installed on power, communication and signal lines can safeguard the electronic equipment from damage and failures and is often a mandatory requirement in lightning protection design and electromagnetic compatibility.
Surge and its sources in building
A surge on power, communication and signal networks can be defined as “a sub-cycle momentary disturbance in the ac waveform evidenced by a sharp and brief discontinuity”. It is an unanticipated change in voltage or current caused by an unpredictable and sometimes unprecedented occurrence. The other terms interchangeably used by the engineering community are – transient, transient surge, impulse, notch, transient burst. The duration of transient voltages may vary from few nano seconds to millisecond. Surges can damage, degrade, or destroy electronic equipment within any office, hospital, commercial building, retail store, or any other facility. Electrical load switching and lightning flashes are the two most common sources of transient surges.
Electrical load switching is a major cause of transient disturbances. Whenever a circuit containing capacitance and inductance is being switched in or out, a transient disturbance occurs because the currents and voltages do not reach their final value instantaneously. This type of disturbance is inescapable and its severity depends on the relative power level of the load being switched and the short-circuit current capacity of the power system. Switching transients can take several forms depending on system configuration and the rate of change on the operating conditions. The causes of switching transients are:
(1) Energizing and de-energizing the lumped and distributed reactive elements in the electrical distribution systems,
(2) Arcing associated with contactor, relay or switch contacts, loose connections and earth faults. These surges can have very destructive effects, even on rugged equipment, and must be controlled at the source and load end.
Though not very frequent as switching surges, lightning flash is the environmentally generated most destructive electrical transient. The major mechanisms by which lightning produces transient voltages in power and signal lines are:
(1) A lightning current pulse due to cloud-to-cloud or cloud-to-ground discharges produce electromagnetic fields that can induce voltages on the conductors of the utility power and signal circuits,
(2) Current flow through earth (GPR), resulting from nearby cloud-to-earth lightning discharges couples into the common earth impedance paths of the earthing network causing Earth Potential Rise (GPR) across two or more earthed points,
(3) A direct lightning strike on high voltage primary circuit couples to secondary of the service transformer by capacitance or by transformer action or both, thus appearing in low voltage ac circuits and
(4) Lightning strikes the secondary circuits directly, resulting very high current and voltage, exceeding the withstand capability of the equipment.
Impact of Surges on electronic equipment
Depending on the severity of the surge and the susceptibility of equipment, three types of degradation and failures are possible: (I) data disruption, (II) hardware stress and latent failure and (III) hardware destruction.
Even very low-magnitude surge can cause data corruption or upset if they occur at the moment of device’s state transition. These data error may be immediately obvious or may only be evident under a unique set of logical conditions that occur occasionally. The secondary effect of a rapid change of current in ground conductors creates transient voltages among different parts of the common ground interconnections. These unwanted transient voltages penetrate into the logic elements causing signal data disruption. In worst case, the noise signal may combine with clock signal and distributed through out the electronic system.
A single lighting or switching transient often cause immediate, but not readily apparent physical damage to semiconductor devices. This damage then finally appears at some time later at which point the failure is obvious. This hardware stress is called “latent failure”. Latent failures relating to ac power and grounding transient conditions are observed in semiconductor devices used for interface equipment or power supplies.
Third possible impact of transients is the immediate and total destruction of hardware components in a single incident. The minimum energy level for destruction of the selected semiconductors are far low comparing to switching and lightning surge energy level. Larger devices, such as signal and power transformers, relay coils, power supply components can also be destroyed.
Surge Protective Device (SPD) for building
Surge protective devices are designed to reduce potentially damaging short-duration transients present on electronic devices. An ideal transient suppression device would be an open circuit at normal voltages, would conduct without delay at some slight voltage above normal, would not allow the voltage to increase during the clamping period, would handle unlimited currents and energy, would revert back to an open circuit when the stress has gone and would never wear out. But unfortunately, till date, there is no single transient suppression device that would meet the above stated ideal conditions. Hence, at present, efficient transient protection requires the use of a number of devices carefully selected to complement each other and thus cover the full range of voltage and current stress condition. Gas Discharge Tube (GDT), Spark gaps, Metal Oxide Varistor (MOV), and Silicon Suppressor diode are the surge suppressor devices mostly used for surge protection.
The GDT and Spark Gaps suppressor devices can handle much larger transient currents. When it strikes, the gas arrestor effectively short-circuits the supply, with only a small voltage being maintained across the electrodes. A disadvantage of the gas arrestor is its relatively low response to an over voltage stress. Another major disadvantage of the GDT is its tendency to remain in a conducting state after the transient condition has ceased. Hence, for this type of devices, it is essential to provide some form of current limiting fast acting fuse or fast acting circuit breaker in the supply line
MOV displays a voltage-dependent resistance characteristic. Below the turnover voltage these devices have high resistance and little circuit loading. When the transient voltage exceeds the turn over voltage, the resistance decreases rapidly and surge current flows in the varistor. The major advantages of the varistor are its low cost and its relatively high transient energy absorption capability. The major disadvantages are progressive degradation of the device with repetitive stress and relatively large slope resistance
PN silicon diode is characterized by their phenomenal surge handling capabilities, extremely fast response time (1x10-12 sec) and low series resistance. The transient voltage suppressor diode has two major advantages, the first being the very high clamping action in a few nano seconds, and the very low slope resistance in the conduction range.
The service entrance SPD has the primary job of intercepting large incoming surges and disposing of them into the building ground. However, some of the surge will be conducted downstream to the appliances in the building, and to other SPDs, either hard-wired or plug-in protectors. “Coordination” is the term used to describe the way in which an incoming surge is apportioned between the first SPD and the downstream SPDs. For the SPDs, the current division is mainly controlled by the relative limiting voltages of the two protectors, and the impedance of the wiring between them. The lower the limiting voltage of the first SPD, and the longer the connecting wiring between the two SPDs, the less surge current will pass to the downstream protector.
LPCI as a complete Solution provider for transient over voltage protection
The sensitive electronics system in a building is prone to damage and failures due to surges on power and signal lines. Switching surges are originated from electrical and electronic systems within the buildings. Lightning surges are better controlled by installing Lightning Protection System (LPS) and Lightning Protection Measure System (LPMS) after making risk assessment. The traditional protective devices like fuse and circuit breakers in electrical distribution of the building does not prevent surges from entering the equipment as their response time is far slower than the fast transient surges. Selection and installation of proper Surge Protective Devices (SPD) at appropriate locations (building entry point and at load end) can prevent failures and reduce down time considerably. A building without a surge protection installation is analogous to a building without a fire extinguisher. The importance of both the systems are felt when the crisis crop-up. The mantra applies here is “prevention is better than cure”. It is worth spending few thousand rupees in installing SPDs to save many lakhs from equipment damage and down time.