Power
The rated power of the relay in kilovolt-amperes, in other words, the maximum apparent load power with which the device can normally operate.
From a physical point of view, kilovolt-amperes and kilowatts mean the same thing, but in kW it is customary to indicate only the active power of the load (see below), and in kVA — full. In AC networks, these powers can vary because active power (the power consumed by resistive-type devices such as heaters) is added to reactive power (the power consumed by capacitors and inductors). The apparent power is calculated from these two quantities.
When selecting control relays for AC networks, it is best to take into account the full power, especially if you plan to connect equipment with electric motors to the device. However, for many modern household appliances, only active power is given in the characteristics — in kilowatts. There are calculation methods that allow converting active power into total power, depending on the type and characteristics of the load; such techniques can be found in special sources.
Rated current
Rated switching current for which the control relay is set. In some models, this parameter can be changed at the request of the user; for such devices, the current value set in the factory settings is indicated (usually, this is the maximum indicator).
For a voltage relay (see “Device”), the rated current is the maximum current allowed for the device and the network segment protected by it for an unlimited time, in fact, the maximum current in normal operation. Exceeding this current is allowed, but not for long (up to several minutes) and not much, not higher than the maximum current (see below). And devices with the functions of a current relay or a power relay can work as follows: if the actual current is between the rated and maximum, the relay goes into the “delayed off” mode, and turns off the power after a certain period of time (for example, 10 minutes). This avoids both overload from long-term operation at high currents, and unnecessary power outages at large, but short-term permissible loads.
Maximum current
Maximum switching current allowed for the control relay. This is the maximum current that the device can safely pass through itself. And for models with current relay functions (see "Device"), this is also the default threshold of instantaneous operation — the current at which the relay instantly turns off the power to the load. In many models, you can set a lower threshold, but not a higher one.
Voltage measuring range
Voltage measurement range supported by the device.
Many modern control relays are equipped with digital displays, which can display, including current voltage; this parameter specifies the operating range of the built-in voltmeter. In this case, the voltage measurement range can be noticeably wider than the difference between the smallest minimum and the largest maximum shutdown threshold (see below). This allows you to use the relay also for online diagnostics of the network status.
For a current relay (see "Device") in this case, the range of operating voltages can be given directly — the voltage at which the device can normally perform its functions.
Measurement accuracy (±)
Measurement accuracy provided by the device. In this case, different types of measurements may be implied, depending on the purpose of the relay (see "Device"). Note that in
power relays and multifunctional devices, the measurement accuracy for voltage and current is usually the same, and a common parameter is given in the characteristics for them.
Accuracy is indicated by the maximum measurement error provided by the device. First of all, the operation accuracy depends on this parameter: the lower the error, the smaller the actual deviations from the specified operation parameters. For modern control relays, an indicator of 3 – 5% is considered acceptable, 1.5 – 3% is not bad, 1 – 1.5% is good, less than 1% is excellent. However, in fact, it is also worth choosing according to this parameter, taking into account how sensitive the connected load is to the accuracy of the specified operation parameters.
Also note that many modern relays are equipped with digital displays that can display various parameters. In such models, the measurement accuracy also determines the accuracy of the readings of such a built-in "tester".
Cut-off time (lower limit)
Time to turn off the device on the lower limit of voltage or current. This is a kind of "reaction time" of the relay: the period of time between reaching the lower limit and turning off the protected network segment.
The lower this value, the more advanced the protection will be, the lower the probability of failure of sensitive devices due to untimely operation of the relay. On the other hand, a high response rate for the lower limit is not as critical as for the upper one, and the shutdown time can be quite long — 1 s or more.
Also note that for some devices, this paragraph gives the minimum turn-off time (fastest response time), while in certain modes this time may be longer. For example, a voltage relay with a lower limit of 160 V may provide tripping after less than 0.05 s when the voltage drops below 120 V and tripping after 1 s when the voltage is in the range of 120 – 160 V, but above 120 V. This avoids unnecessary shutdowns with relatively weak and short-term voltage deviations. In the characteristics of such a device, 0.05 s will be indicated.
Off time (upper limit)
The device shutdown time on the upper limit of voltage or current. This is a kind of "reaction time" of the relay: the period of time between reaching the upper limit and turning off the protected network segment.
The lower this value, the more advanced the protection will be, the lower the probability of failure of sensitive devices due to untimely operation of the relay. Note that a short reaction time in this case is especially important, because too high a voltage or current is a serious danger to any device.
Reclosing delay
The reclosing delay is the time after a safety shutdown after which the device turns on the mains power again. Usually, in modern control relays this time can be adjusted, so the characteristics indicate the range from the minimum to the maximum value.
Adjustment of the delay allows you to adjust the format of the relay to the characteristics of the network. So, if voltage failures do not mean critical problems, you can set the minimum re-closing time, and if jumps occur only with serious problems that need to be fixed, it is better to turn on the maximum delay. Note that triggered relays usually allow manual reclosing, and in most models it is possible to completely disable the automatic reclosing function.
Lower cut-off limit
The lower voltage shutdown limit provided in the relay; when the voltage drops below this value, the device turns off the power to the protected network segment. Usually, this limit can be adjusted, and the specifications indicate the range of such adjustment.
Although the main danger for various electrical appliances is increased voltage, many devices do not tolerate too low voltage. Therefore, modern voltage relays provide shutdown not only on the upper, but also on the lower threshold.
