ABB CM Range Monitoring Relays: Full Range Review
What is the ABB CM range of monitoring relays? The CM range is ABB's family of DIN-rail measuring and control relays, each 22.5 mm wide, that watches one electrical or physical quantity against an adjustable threshold and switches a change-over output when it drifts out of the set window, per IEC 60255 and IEC 60947-5-1. Pick the wrong sub-family and you either miss the fault mode entirely — a CM-ESS current relay will not see a lost phase — or pay for functions the panel does not need. This review walks the range function by function: three-phase supply (CM-MPS, CM-MPN), single-phase current (CM-ESS, CM-SRS, CM-EFS), liquid level (CM-ENS, CM-ENE), temperature and thermistor (CM-TCS, CM-MSS, CM-MSN), and combined voltage/frequency (CM-UFD, CM-UFS).
The CM Range at a Glance
Every CM relay shares the same physical language: 22.5 mm DIN-rail housing, screw or spring terminals, one or two change-over (c/o) output contacts, and a front-panel dial or DIP switches for threshold and delay. What changes between series is the measured quantity and, on the higher models, whether the relay needs its own control-supply terminals or draws power straight from the line it is watching. That last point decides how many terminals end up in the panel and whether a control-transformer fault can also blind the monitoring function.
Fault memory (latching the trip until manually reset) and true-RMS vs average measurement separate the base models from the higher-end ones in each sub-family. On a supply with harmonics from VFDs, average-sensing relays under-read the true disturbance; true-RMS models catch it. That distinction matters more on current and voltage monitoring than on simple phase-sequence checks, where the relay only needs to see rotation, not waveform shape.
Three-Phase Supply Monitoring: CM-MPS and CM-MPN
CM-MPS and CM-MPN cover phase sequence, phase loss, asymmetry, and over/undervoltage in one 22.5 mm block — the same function set as a phase sequence and phase failure relay from any brand, just packaged under ABB's CM naming. CM-MPS needs no auxiliary supply; it draws its own power from the three measured lines, which is why it turns up on motor starters where a control transformer was never installed for anything else. Wrong rotation on a three-phase motor means it turns backwards on start — a pump running in reverse, a conveyor feeding the wrong way. These relays block the start before the contactor ever pulls in.
Asymmetry tripping is the setting installers get wrong most often. Set the window too tight and a slightly unbalanced but healthy supply nuisance-trips the line during every cold morning peak load. Set it too loose and a genuine single-phasing fault — one blown fuse upstream — goes undetected until a motor winding overheats.
Formula: Voltage Asymmetry — Source: IEC 60947-5-1, general monitoring-relay practice
Asymmetry% = (Vmax deviation / Vaverage) × 100
| Symbol | Description | Unit |
|---|---|---|
| Vmax deviation | Largest difference between one phase voltage and the three-phase average | V |
| Vaverage | Mean of the three measured line voltages | V |
| Asymmetry% | Resulting imbalance figure compared against the relay's set threshold | % |
Single-Phase and Current Monitoring: CM-ESS, CM-SRS, CM-EFS
CM-ESS and CM-SRS monitor single-phase current, over or under, through a built-in shunt or an external current transformer on higher-range models. CM-EFS extends this to a broader current range for larger loads. Undercurrent catches what overcurrent misses entirely: a snapped drive belt, a pump running dry, a conveyor with the load fallen off — the motor still draws current, just not enough to trip a thermal element. Overcurrent on the same relay catches a mechanical jam before it works its way up to a genuine overload trip.
What we see in the field: CM-ESS gets specified on pump skids as a cheap belt-break and dry-run backup behind the main thermal overload relays, because the thermal element alone will not see a broken coupling until the motor has been running unloaded for a while and heat has built somewhere else in the drivetrain.
Liquid Level Monitoring: CM-ENS and CM-ENE
CM-ENS and CM-ENE use conductive sensing — a low voltage across probe electrodes reads whether the liquid bridges the gap between them. Sensitivity, given in kOhm, is adjustable to match the liquid's own conductivity; tap water and lightly contaminated process water need very different settings than a highly conductive tank. Wire three probes (low, high, common) for pump up/down control, or a single probe against the tank wall for simple dry-run cutoff. A liquid level monitoring relay of this type has no moving parts in the tank, unlike a float switch, which matters where the liquid carries solids that would jam a float.
Temperature and Thermistor Monitoring: CM-TCS, CM-MSS, CM-MSN
CM-TCS, CM-MSS, and CM-MSN cover the two temperature-sensing methods used in motor and process protection. PTC thermistor input trips when the sensor embedded in the winding hits its reference resistance around 3.3 kOhm, per IEC 60947-8 — a binary go/no-go signal, not a temperature reading. PT100/PT1000 input on the higher models reads an actual resistance-to-temperature curve and switches at a set point the panel builder chooses, useful where the process needs a real number, not just a trip. Motor-protection specs that call for thermistor motor protection alongside a mechanical overload usually mean this class of relay sitting behind the starter's motor protection circuit breakers, catching winding heat that a current-based device cannot see directly.
Formula: PTC Trip Reference — Source: IEC 60947-8
Rtrip ≈ 3.3 kΩ (nominal reference resistance)
| Symbol | Description | Unit |
|---|---|---|
| Rtrip | PTC sensor resistance at which the relay output changes state | kΩ |
| Rcold | Typical PTC resistance well below the trip point (cold winding) | Ω |
This depends on how the thermistor chain is wired: PTC sensors in series inside a winding sum their resistance, so a relay set for a single-sensor reference can misread a three-sensor chain unless it is rated for it. Check the wiring diagram against the relay's rated sensor count before commissioning, not after.
Voltage and Frequency: CM-UFD and CM-UFS
CM-UFD and CM-UFS combine voltage and frequency monitoring in one relay — over/undervoltage plus over/underfrequency, useful behind a generator or an inverter output where both quantities drift together during a fault. A generator losing governor control sags in frequency before voltage collapses; catching frequency first buys the transfer switch logic a few extra cycles to act. These sit less often on a straight utility feed, where frequency is tightly regulated, and more often near an monitoring and control relays shortlist for standby power and islanded generation.
Choosing Within the CM Range
Start from the fault mode, not the brand. Phase loss and wrong rotation point to CM-MPS/CM-MPN. A broken belt or dry pump points to CM-ESS/CM-SRS. A winding temperature limit points to CM-TCS/CM-MSS/CM-MSN. Once the function is fixed, the remaining choice is aux-supply need, output contact count (one or two c/o), and whether fault memory (latching) is required — a latched trip forces an operator to walk to the panel and reset, which some sites want for anything that stops a process, and others find a nuisance for a low-consequence fault.
Where a CM relay drives a starter directly rather than a PLC input, cross-check its output rating against the contactors coil it switches; most CM outputs are rated for small control loads, not for switching a large coil directly without an interposing relay.
| Criteria | CM-MPS (ABB) | CM-ESS (ABB) | CM-TCS (ABB) |
|---|---|---|---|
| Function | 3-phase sequence, loss, asymmetry, over/undervoltage | Single-phase over/undercurrent | PTC thermistor motor temperature |
| Auxiliary supply | Not required — self-powered from measured lines | Required on most models | Required |
| Typical use | Motor starter phase protection | Belt-break, dry-run backup | Winding over-temperature |
| Output | 1-2 c/o contacts | 1-2 c/o contacts | 1 c/o contact |
Frequently Asked Questions
Which ABB CM relay handles three-phase phase-loss and sequence protection?
CM-MPS and CM-MPN cover phase sequence, phase loss, and asymmetry in a single relay. CM-MPS additionally needs no auxiliary supply, drawing power directly from the three-phase lines it measures.
Does the CM-MPS need a separate control-supply input?
No. CM-MPS is self-powered from the measured three-phase supply, which removes the need for a control transformer purely to feed the monitoring relay.
What is the difference between CM-ESS and CM-SRS current relays?
Both monitor single-phase current for over- or undercurrent conditions. They differ in current range and sensing method (built-in shunt vs external CT compatibility) rather than the core function, so the choice comes down to the load's rated current and available panel space.
Can a CM-TCS thermistor relay be wired directly to a motor's PTC sensor?
Yes — CM-TCS reads the PTC chain embedded in the motor winding and trips at the reference resistance defined in IEC 60947-8. Confirm the relay's rated sensor count matches the number of PTC elements wired in series inside the motor.
How many output contacts does a typical CM relay provide?
Most CM relays provide one or two change-over (SPDT/DPDT) contacts. The exact count depends on the specific model within each sub-family, so check the datasheet against whether the application needs to drive both a contactor coil and a separate alarm circuit.
Do CM level relays work in oil or non-conductive liquids?
No. CM-ENS and CM-ENE rely on conductive sensing between probe electrodes, which requires the liquid to carry current. Non-conductive media need a float, capacitive, or ultrasonic level device instead.
Conclusion
The CM range is organized by function first, aux-supply need and output count second. Start from what has to be caught — lost phase, broken belt, dry pump, hot winding, sagging frequency — and the sub-family picks itself: CM-MPS/CM-MPN for three-phase supply, CM-ESS/CM-SRS/CM-EFS for current, CM-ENS/CM-ENE for level, CM-TCS/CM-MSS/CM-MSN for temperature, CM-UFD/CM-UFS for combined voltage and frequency. For the wider picture across brands and functions, the monitoring relay engineering guide covers threshold, hysteresis, and delay settings that apply across the whole CM range.
