- The nature of surge voltages and their effect on technology
- How the SPD works and how it works
- What is the difference between protection classes
- Is there a need for an SPD, risk assessment
- Installation of devices in the main switchboard
If your house has a lot of expensive household appliances, it is better to take care of the organization of comprehensive protection of the electrical network. In this article we will tell you about surge protection devices, why they are needed, what are they and how they are installed.
The nature of surge voltages and their effect on technology
Since childhood, many people are familiar with the fuss of disconnecting household electrical appliances from the network at the first sign of an impending thunderstorm. Today, the electrical equipment of urban networks has become more sophisticated, which is why many neglect basic protection devices. At the same time, the problem has not completely disappeared, household appliances, especially in private houses, are still at risk.
The nature of the occurrence of impulse overvoltages (IP) can be natural and man-made. In the first case, IP arise due to lightning hitting overhead power lines, and the distance between the point of impact and the consumers at risk can be up to several kilometers. A strike is also possible on radio masts and lightning rods connected to the main grounding circuit, in which case an induced overvoltage appears in the household network.
1 – remote lightning strike in power lines; 2 – consumers; 3 – ground loop; 4 – close lightning strike in power lines; 5 – direct lightning strike to a lightning rod
Man-made IPs are unpredictable; they arise as a result of switching overloads at transformer and distribution substations. With an asymmetric increase in power (only on one phase), a sharp voltage surge is possible, it is almost impossible to foresee this.
Impulse voltages are very short in time (less than 0.006 s), they appear in the network systematically and most often pass unnoticed by the observer. Household appliances are designed to withstand overvoltages up to 1000 V, which appear most often. At a higher voltage, failure of the power supplies is guaranteed, insulation breakdown in the house wiring is also possible, which leads to multiple short circuits and fire.
How the SPD works and how it works
SPD, depending on the protection class, can have a semiconductor device on varistors, or have a contact arrester. In normal mode, the SPD operates in bypass mode, the current inside it flows through the conductive shunt. The shunt is connected to the protective ground through a varistor or two electrodes with a strictly regulated gap.
With a voltage jump, even a very short one, the current passes through these elements and spreads along the ground or is compensated by a sharp drop in resistance in the phase-zero loop (short circuit). After the voltage stabilizes, the arrester loses its capacity, and the device works in normal mode again..
Thus, the SPD closes the circuit for some time so that the excess voltage can be converted into heat energy. At the same time, significant currents pass through the device – from tens to hundreds of kiloamperes.
What is the difference between protection classes
Depending on the causes of IP, two characteristics of the overvoltage wave are distinguished: 8/20 and 10/350 microseconds. The first digit is the time it takes for the MT to reach its maximum value, the second is the time it takes to fall to nominal values. As you can see, the second type of overvoltage is more dangerous..
Class I devices are designed for protection against power supply with a characteristic of 10/350 μs, which most often occur during a lightning discharge in a power transmission line closer than 1500 m to the consumer. The devices are capable of passing a current of 25 to 100 kA through themselves for a short time, almost all Class I devices are based on arresters.
Class II SPDs are designed to compensate for power supplies with a characteristic of 8/20 μs, peak current values in them vary from 10 to 40 kA.
Protection class III is designed to compensate for overvoltages with current values less than 10 kA with a power supply characteristic of 8/20 μs. Devices of protection class II and III are based on semiconductor elements.
It may seem that it is enough to install only Class I devices, as the most powerful, but this is not so. The problem is that the higher the lower threshold of the throughput current, the less sensitive the SPD is. In other words: with short and relatively low values of the power supply, a powerful SPD may not work, and a more sensitive one will not cope with currents of this magnitude..
Devices with protection class III are designed to eliminate the lowest power supplies – only a few thousand volts. They are completely similar in characteristics to protection devices installed by manufacturers in power supplies for household appliances. With a redundant installation, they are the first to take on the load and prevent the operation of an SPD in devices whose resource is limited to 20-30 cycles.
Is there a need for an SPD, risk assessment
A complete list of requirements for the organization of protection against power supply is set out in IEC 61643-21, it is possible to determine the mandatory installation according to the IEC 62305-2 standard, according to which a specific assessment of the degree of risk of lightning strike and the consequences caused by it is established.
In general, when supplying power from overhead transmission lines, the installation of class I SPDs is almost always preferable, unless a set of measures has been taken to reduce the impact of thunderstorms on the power supply mode: re-grounding of supports, PEN-conductor and metal bearing elements, a lightning rod with a separate ground loop, installation potential equalization systems.
An easier way to assess risk is to compare the cost of unprotected appliances and security devices. Even in multi-storey buildings, where overvoltages are very low with a characteristic 8/20, the risk of insulation breakdown or failure of devices is quite high..
Installation of devices in the main switchboard
Most SPDs are modular and can be installed on a 35 mm DIN rail. The only requirement is that the shield for installing the SPD must have a metal case with a mandatory connection to the protective conductor..
When choosing an SPD, in addition to the basic performance characteristics, you should also take into account the rated operating current in bypass mode, it must correspond to the load in your mains. Another parameter is the maximum limiting voltage, it should not be lower than the highest value within the daily fluctuations.
SPDs are connected in series to a single-phase or three-phase supply network, respectively, through a two-pole and four-pole circuit breaker. Its installation is necessary in case of soldering the spark gap electrodes or breakdown of the varistor, which causes a permanent short circuit. Phases and a protective conductor are connected to the upper terminals of the SPD, to the lower terminals – zero.
Example of SPD connection: 1 – input; 2 – automatic switch; 3 – SPD; 4 – grounding bus; 5 – ground loop; 6 – electricity meter; 7 – differential machine; 8 – to consumer machines
When installing several protective devices with different protection classes, they must be coordinated using special chokes connected in series with the SPD. Protective devices are built into the circuit in increasing class. Without coordination, more sensitive SPDs will take on the main load and fail earlier..
The installation of chokes can be avoided if the length of the cable line between the devices is more than 10 meters. For this reason, class I SPDs are mounted on the facade even before the meter, protecting the metering unit from overvoltage, and the second and third class are installed, respectively, on the ASU and floor / group shields.