An undervoltage relay, ANSI device number 27, watches the voltage on a bus or at a load and acts when that voltage falls below a set threshold for longer than a set time. Its job is to protect equipment that cannot run safely on low voltage, most often motors, and to bring a facility back up in an orderly way after the voltage recovers. The trick is in the time delay: set too fast, it trips on every harmless utility dip; set with a sensible delay, it rides through the sags that recover on their own and only acts on the ones that do not.
Undervoltage Relay (27) in one line: An undervoltage relay (ANSI 27) monitors system voltage and trips or blocks connected loads when the voltage drops below a preset level for a preset time. It protects motors and other equipment from the damage and stalling that low voltage causes, and it controls how loads are dropped and restarted around a voltage disturbance.
Low voltage is quietly hard on rotating equipment. An induction motor's torque falls off roughly with the square of the voltage, so a deep sag can leave a loaded motor unable to keep turning; it slows, draws heavy current trying to hold speed, and overheats. If enough motors on a bus stall at once, they pull the voltage down further and can drag the whole bus into a collapse. The undervoltage relay is the device that decides, on behalf of those loads, when the voltage has gotten low enough that staying connected does more harm than good.
Beyond protecting the motor itself, device 27 governs what happens on the way back. Many starters and contactors drop out on their own when voltage disappears, but if they were allowed to snap all their loads back on the instant voltage returned, the simultaneous inrush could pull the bus straight back down. An undervoltage scheme lets a facility drop selected loads on the sag and then restart them in a controlled sequence, so the system recovers instead of chattering.
Undervoltage protection also serves as a permissive and interlock. It can block a large motor from starting into an already weak bus, confirm that a source is actually dead before a transfer or tie breaker closes onto it, and provide the low-voltage sensing that automatic transfer and load-shedding schemes are built around. In each case the 27 element is the piece that answers a simple question with confidence: is the voltage here, or is it not.
A device 27 element has at least two key settings: the voltage pickup, the level below which it considers the bus undervoltage, and the time delay, how long the voltage must stay below that level before the relay acts. The delay is what separates a nuisance trip from real protection. Utility faults elsewhere on the grid cause brief voltage sags that clear when the remote fault is cleared, often within a fraction of a second, and the local bus recovers on its own. A relay with no delay would trip a facility offline for every one of those, so the delay is deliberately set to ride through the short dips and act only on the sustained ones.
Ride-through is the design goal of tuning that delay. The engineer looks at how deep and how long a sag the connected loads can actually tolerate, and at how long typical grid disturbances last, then sets the pickup and delay so the relay ignores the recoverable events and catches the genuine loss of voltage. Sensitive drives may need their own ride-through capability layered on top, but the 27 relay sets the coarse policy for the bus as a whole.
Complementing the trip logic, undervoltage schemes usually include restart control. A relay may hold a lockout until voltage has been restored and stable for a set time, then release loads in stages. On motors that must not restart unexpectedly, the scheme can require a manual reset so an operator confirms the equipment is safe before it turns again. These behaviors, layered on the basic pickup and delay, are what turn a raw voltage measurement into a coordinated response that a facility can survive without cascading.
In a modern facility the 27 element usually lives inside a multifunction protection relay, and its state, pickup, and trip are all available as data. That matters because a voltage sag is often the first symptom of a larger problem, whether an incoming utility disturbance, an overloaded transformer, or a fault on a parallel feeder. Capturing when the voltage dipped, how far, and for how long turns a vague report of tripped motors into a clear timeline.
Undervoltage sensing is also the trigger for load shedding on oilfield power systems that mix utility supply with local generation. When voltage or frequency starts to sag because generation is short of load, an undervoltage or underfrequency scheme sheds preassigned, lower-priority loads to keep the critical ones supplied. Coordinating which loads drop and in what order is the difference between shedding a few heaters and blacking out a production site.
A cloud SCADA platform such as Merobix reads those voltage measurements and relay statuses from the PLCs and protection devices at each site and trends them over time. Operators can see a sag as it happens, get an alarm the moment a 27 element trips or a load sheds, and pull the historized voltage record afterward to understand the event. For remote and unmanned sites the payoff is direct: instead of learning that a bus dipped when production goes missing, the crew is notified at the moment of the sag with the numbers already in hand.
In the ANSI/IEEE device numbering standard, 27 is the number assigned to an undervoltage relay. When you see 27 on a single-line diagram or a relay, it means an element that operates when voltage falls below a set value. The same numbering scheme uses other numbers, such as 50 for instantaneous overcurrent and 32 for directional power, so device numbers give a compact shorthand for a protection function.
The time delay lets the relay ride through short voltage sags that recover on their own, such as those caused by faults elsewhere on the utility grid. Without a delay, the relay would trip the facility offline for every brief dip, which is a nuisance rather than protection. The delay is set so the relay ignores recoverable sags and acts only on a sustained loss of voltage that the equipment cannot tolerate.
A motor's torque falls sharply as voltage drops, so on a deep sag a loaded motor can stall, draw heavy current, and overheat. An undervoltage relay disconnects the motor before that damage occurs and controls when it is allowed to restart. That prevents both the immediate thermal stress and the uncontrolled inrush that would follow if every motor snapped back on the instant voltage returned.
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