Monitoring Relay vs Protection Relay vs PLC Monitoring
What is the difference between a monitoring relay, a protection relay, and PLC-based monitoring? A monitoring relay is a DIN-rail device built to IEC 60255 and IEC 60947-5-1 that watches one quantity against an adjustable threshold, hysteresis band, and trip delay, then switches an output contact. A protection relay is a numeric, IEC 60255 protection-class device that bundles several fault functions with trip curves and usually a communication port. PLC-based monitoring performs the same comparison in software, reading the signal through an I/O card and acting on program logic instead of a dedicated relay. Get the choice wrong and a panel either pays for functions it never uses or loses the trip-curve documentation a feeder actually needs. This article covers what each device does, where the three overlap, how to read a spec sheet for the difference, and when a panel builder should combine two of them instead of picking one.
What Is a Monitoring Relay?
A monitoring relay — also called a measuring and control relay — measures one electrical or physical quantity continuously and compares it to a set window. Cross the threshold, and after the trip delay expires, the output contact changes state. It does not break the main power circuit; the contact drives a contactor coil, a PLC digital input, or an alarm horn. Width is typically 17.5 mm to 22.5 mm, DIN-rail mounted, with 1-2 change-over outputs. Function is narrow by design: a three-phase supply relay checks sequence, loss, and voltage window; a level relay checks conductivity between probes; a thermistor relay checks PTC resistance against the IEC 60947-8 reference. For the full functional breakdown across quantities, see the monitoring relay engineering guide, and for how the device families split by measured quantity, see types of monitoring relays.
What Is a Protection Relay?
A protection relay is a numeric device built to the IEC 60255 protection class. One unit typically evaluates several fault conditions at once — overcurrent, earth fault, thermal overload, undervoltage — each against its own trip curve, and issues a trip command straight to a breaker's shunt trip or undervoltage release. Most units log events with a timestamp and expose a communication port (commonly Modbus) for SCADA integration. Where a monitoring relay's output is a general-purpose contact rated for pilot duty, a protection relay's trip output is engineered specifically to open the associated breaker, and coordination studies size its curve against upstream and downstream devices. This is why feeder protection at a switchboard incomer sits on a protection relay, not a monitoring relay, even though both compare a measured value to a threshold.
What Is PLC-Based Monitoring?
PLC-based monitoring reads the same signal (voltage, current, level, temperature) through an analog or digital input card, then compares it to a threshold written in the program instead of set on a dial. Nothing about the sensing changes; a current transformer feeding a PLC analog input measures the same current a monitoring relay's built-in shunt would. What changes is where the logic lives. A threshold change on a monitoring relay is a knob turn on the device face. A threshold change on a PLC is a program edit, a download, and, depending on the site's change-control process, a re-test before it goes live. What we see in the field: panels that already run a PLC for sequencing often fold in one more comparison rather than add a dedicated relay, because the I/O and program structure already exist. Panels with no PLC at all get a dedicated relay every time, since installing a PLC just to watch one voltage window costs more in hardware and engineering hours than the relay itself.
Monitoring Relay vs Protection Relay: What Actually Separates Them
Both devices measure a quantity and compare it to a threshold. Both use hysteresis to stop the output from chattering near the setpoint. The split is in scope, output rating, and documentation. A monitoring relay typically handles one function family; a protection relay stacks several protection curves — overcurrent, earth fault, thermal — behind one numeric core, with each curve independently timed and coordinated against the rest of the switchboard. A monitoring relay's contact is general-purpose, pilot-duty; a protection relay's trip output is purpose-built to open a specific breaker and is part of a documented protection scheme. Cost follows scope: a single-function monitoring relay costs a fraction of a multifunction numeric protection relay, which is exactly why LV distribution boards use monitoring relays for phase loss and voltage windows, while MV switchgear and large motor feeders use protection relays for the same underlying comparison done at higher stakes.
Formula: Reset Threshold from Hysteresis — Source: general monitoring relay setting practice (IEC 60947-5-1 context)
Vreset = Vtrip × (1 − h)
| Symbol | Description | Unit |
|---|---|---|
| Vtrip | Set (trip) threshold | V or A |
| h | Hysteresis, expressed as a fraction (e.g. 0.05 for 5%) | ratio |
| Vreset | Value at which the output re-energizes once the fault clears | V or A |
This formula holds on both device types. Set hysteresis too tight on either one and the output chatters every time supply noise brushes the threshold; set it too wide and a real fault sits unflagged longer than the panel builder expects. On a monitoring relay this is a dial. On a protection relay it is one parameter among dozens in the curve set, which is part of why commissioning a numeric protection relay takes longer than wiring in a monitoring relay and turning two knobs.
Monitoring Relay vs PLC Monitoring: Hardwired vs Programmable
A monitoring relay works the moment it is powered and wired: no program, no HMI, no download. That is its strength in a small panel: fewer failure points, no software to maintain, a technician with a screwdriver can adjust the setpoint on site. A PLC brings flexibility the relay cannot match: combine three inputs into one alarm, delay the output based on another machine state, log the trend to a historian, or change the logic without opening the panel. The tradeoff is dependency — a PLC fault, a corrupted program, or a communication loss can take the monitoring function down with it, where a dedicated relay keeps working independent of everything else in the cabinet. Some panel builders standardize on relays for anything safety-adjacent (phase loss on a motor, insulation monitoring on an IT system) and reserve the PLC for monitoring that feeds a process decision rather than a protective one. This depends on the plant's change-control maturity, since a site with disciplined PLC program versioning tolerates more logic in software than one where edits happen ad hoc on the factory floor.
For motor circuits specifically, this split matters at the coil. A monitoring relay wired to the contactor's control circuit, paired with a motor protection circuit breaker for the short-circuit and overload trip, gives two independent protection layers that do not depend on the same PLC scan cycle. See monitoring relays for motor protection and thermistor input for how phase monitoring and PTC thermistor sensing combine on a motor starter, and MPCB phase loss and single-phasing protection for the breaker side of the same fault.
Side-by-Side Comparison
| Criteria | Monitoring Relay | Protection Relay | PLC Monitoring |
|---|---|---|---|
| Function scope | One or two related quantities per device | Multiple protection functions bundled in one numeric relay | Unlimited, bounded by I/O count and program |
| How thresholds are set | Dial, DIP switch, or built-in keypad | Keypad, HMI, or engineering software with curve library | Written into the program, changed by re-download |
| Governing standard | IEC 60255 (measuring) / IEC 60947-5-1 | IEC 60255 protection class | IEC 61131-3 for the program; sensor per its own standard |
| Output | SPDT/DPDT pilot contact to contactor, alarm, or PLC input | Trip command engineered to open a specific breaker | Digital output on an I/O card, logic-defined |
| Typical placement | LV distribution board, motor starter, pump panel | MV switchgear, incoming feeder, large motor | Anywhere a PLC already exists in the control scheme |
| Effort to change a setpoint | Turn a dial, no program change | Edit a curve parameter in relay software | Edit program, test, re-download |
How to Choose the Right Approach for a Panel
Start with what the output has to do. If it needs to open a breaker with a documented, coordinated curve, specify a protection relay — nothing else on this list is built for that job. If it needs to drive a contactor coil, an alarm, or a PLC input off one measured quantity, a monitoring relay from the monitoring and control relays range covers it at a fraction of the cost and commissioning time of a numeric relay. If the decision depends on combining several signals, or the panel already runs a PLC for sequencing, fold the comparison into the program instead of adding hardware. Most real panels use two of the three together: a monitoring relay for phase loss on the incoming supply, a PLC for process-level thresholds like tank level and flow, and, only where a feeder or motor genuinely needs coordinated fault clearing, a protection relay. Do not default to a protection relay for every trip decision; it adds commissioning time and cost that a simple phase-loss or voltage-window function does not need. Likewise, do not default to a PLC for a single protective function that has to keep working when the PLC does not.
Frequently Asked Questions
Is a monitoring relay the same as a protection relay?
No. A monitoring relay measures one quantity and switches a general-purpose pilot contact; a protection relay is a numeric, IEC 60255 protection-class device that bundles multiple fault functions with coordinated trip curves and issues a trip command engineered for a specific breaker.
Can a PLC replace a monitoring relay?
For process-level thresholds, yes — a PLC analog input reading the same sensor can do the comparison in software. For protective functions like phase loss or insulation monitoring, most panel builders keep a dedicated relay so the function keeps working independent of the PLC's program or communication state.
Does a monitoring relay interrupt the circuit itself?
No. Its output contact drives a contactor coil, alarm, or PLC input; the contactor or breaker does the actual interruption. The relay's contact is rated for pilot duty, not for breaking fault current.
When should I use a protection relay instead of a monitoring relay?
When the circuit needs a documented, coordinated trip curve — typically MV switchgear, incoming feeders, or large motors where selectivity studies size each device's timing against the others on the network.
Can a monitoring relay and a PLC work together in the same panel?
Yes, and this is common. The monitoring relay handles a protective function directly at the contactor coil, while the PLC reads a separate signal for process control or logs the relay's contact state for SCADA visibility.
Conclusion
None of these three replaces the other two. A monitoring relay is the cheapest, fastest way to watch one quantity and drive a contactor or alarm. A protection relay is the only option once a breaker needs a documented, coordinated trip curve. A PLC earns its place when the decision needs several signals combined or when the panel already has the I/O and program structure to absorb one more comparison. Specify by consequence, not by habit — the cost of over-specifying a protection relay for a simple phase-loss function is wasted budget; the cost of under-specifying a monitoring function onto a PLC that shares a program with everything else in the panel can be a fault nobody catches until the motor already single-phased.
