— Voltage relay. Protective devices that automatically turn off the protected area when the mains voltage goes beyond the specified parameters. Usually, such devices are able to respond to both a significant increase and a significant decrease in voltage. A voltage relay will be a useful addition to traditional protection such as plugs or a circuit breaker, since such protection only reacts to excess current and does not monitor voltage. And in three-phase networks, protective relays can also provide monitoring of phase synchronization, triggering in case of skew, breakage, sticking or phase sequence violation.
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Current relay. Protective devices that automatically turn off the protected area when the consumed current exceeds the specified parameters. It is similar in purpose to circuit breakers, however, firstly, the current relays can also respond to a decrease in current, and secondly, such a device may not work instantly, but after a certain time. It makes sense to install a current relay where short-term operation at high currents is allowed, however, the time of this operation must be limited, and also where long-term idle operation at low currents is undesirable. A classic example is connecting a power-controlled motor: the current at maximum power can be set as an upper limit, and the lower limit can be set just above the no-load current. The shutdown time in such devices, usually, can be set within a few minutes.
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— Power relay. Protective devices that automatically turn off the protected section of the circuit when the power consumption is exceeded. Such a device monitors both the current strength and the voltage at the same time — we recall that the power is calculated by the formula "current strength multiplied by voltage." The need for a power relay is due to the fact that in some situations it is not an excess of current or voltage that gives an overload, but a combination of them — moreover, both volts and amperes can remain within acceptable values.
— Multifunctional relay. Models that combine the capabilities of several protective devices. A multifunctional device usually has the function of a voltage relay (with upper and lower trip limits) and a power relay or current relay (both with only an upper limit). See above for details of each variety; here we note that their multifunctional relay allows you to get by with one device instead of two.
— Phase selection relay. Protective devices used when supplying a single-phase load through a three-phase network. As the name suggests, such a relay provides automatic selection of the most favorable phase; in other words, if a failure occurs on the current phase, the device switches the load to another phase, with more stable voltage indicators. Such devices are mainly intended for connecting especially important and sensitive devices that have increased requirements for voltage stability.
— Impulse relay. Control devices that close or open the circuit when a short-term voltage pulse is applied to the control input. Also, such devices are called bistable, since each of the switch positions (both “on” and “off”) is stable and changes only when a control pulse is received. One of the options for using such relays is to control lighting from several places at the same time, for example, from two switches installed at different ends of a long corridor. By connecting both of these switches to an impulse relay, one can turn on the light at the entrance to the corridor, and the second one can turn it off at the exit, regardless of the direction of movement. Theoretically, even on one relay, you can tie up as many switches as you like; there are more complex schemes for the use of bistable control devices.Width expressed in unit units. This unit is used to measure the width of rack-mounted devices and makes it easy to estimate how much space you need. Width in unit is always expressed as whole numbers, 1 unit is approximately equal to 17.5 mm. There are models
1U,
2U,
3U and more
The upper current shutdown limit provided in the device is the threshold above which the relay switches off the protected network segment. Many models allow you to adjust this limit; for them, the range of such adjustment is indicated in the characteristics. Note that for the upper limit of this range, some manufacturers take not the maximum, but the rated current of the device; see above for details on these options.
