ABB / CM measuring and monitoring relays

On the current monitoring relays the measuring range is selected by which terminal you wire to, not by a switch. Wire to the wrong one and the relay is reading the wrong scale.

The CM range covers voltage, current, phase, insulation, temperature, motor load and thermistor monitoring. These sit in most motor control centres and are the sort of thing replaced without much thought until the replacement behaves differently.

The range is selected by terminal

Start here, because it is the least obvious thing in the range.

On the CM-SRS.M current monitoring relays, ABB specifies that the measuring range is selected by connecting the monitored wire to the corresponding terminal B1, B2 or B3 against C.

One CM-SRS.M2 covers three current ranges, and the wiring decides which.

That is good for stocking, since one part covers 0.3 A to 15 A. It also means that if a relay is misreading after a rewire, the terminal is the first thing to check, and there is no switch position to give the mistake away.

What is adjustable, and how far

The CM range is configured with DIP switches and front-face potentiometers rather than software, which makes replacement straightforward if you record the settings first.

DIP switch selections

Potentiometer adjustments, with direct reading scales

Timing in progress is shown by a flashing green LED.

Why the start-up delay matters

The start-up delay TS is the setting that stops nuisance trips during motor starting.

A motor drawing six or seven times full load current at start will trip an overcurrent relay set for running conditions. The start-up delay holds the relay off until the motor is up to speed.

So if a monitoring relay trips every time a motor starts, the answer is usually TS rather than a different relay. And a replacement relay arrives with the potentiometers at default, which is why a like-for-like swap can start nuisance tripping.

Photograph the settings before you remove the old one. The scales are direct reading, so a photo is all it takes.

Window monitoring is a different function

A distinction worth understanding before substituting.

ABB describes the CM-EFS.2 as a voltage window monitoring relay for simultaneous monitoring of overvoltage and undervoltage in single-phase AC and DC systems.

Depending on configuration, one changeover contact each can be used for over and under, or both contacts in parallel. The measured voltage is applied to terminals B and C.

A CM-ESS is not a substitute for a CM-EFS. If the application needs both limits watched at once, a relay configured for one of them leaves the other unprotected.

Reading a CM designation

Take CM-PVS.41S, order code 1SVR730794R3300.

Confirmed specification for the CM-PVS.41S: rated control supply and three-phase measuring voltage of 300 to 500 V AC, with a 2 changeover output rated 250 V, 4 A, monitoring over and undervoltage and phase failure.

The trailing digits are the voltage variant

The CM-PVS is published as CM-PVS.31, CM-PVS.41 and CM-PVS.81, and the CM-PSS as CM-PSS.31 and CM-PSS.41.

Those digits are supply and measuring voltage ranges, not model generations. A .31 and a .41 are the same function on different voltages, so the digits must match your supply.

Send us the full designation including the digits and any trailing letter.

The function letters

The trailing S, N and E vary across the range. Rather than guess at the convention, send us the exact designation off the relay and we will match it. The letter is part of the part number, not a description.

The monitoring categories

Single-phase current and voltage

CM-SRS and CM-SRN monitor AC and DC currents. CM-ESS, CM-ESN and CM-EFN monitor voltage.

The CM-ESS.M can be configured for over or under monitoring, with hysteresis adjustable across the same 3 to 30 percent of threshold.

Three-phase monitoring

Phase, phase sequence and phase unbalance are covered by CM-PBE, CM-PVE, CM-PFE, CM-PFS, CM-PFN, CM-PVN, CM-ASS, CM-ASN and CM-MPS.

ABB describes the CM-PVS as monitoring phase sequence, phase failure, over and undervoltage, with adjustable thresholds. If all three phases are present at correct voltage, the output relays are energised.

That is the normally energised arrangement, so a loss of supply to the relay itself also drops the output. Worth knowing when interpreting a trip.

Phase sequence monitoring matters on any three-phase motor that must not run backwards. Pumps, conveyors and compressors all qualify, and a relay that only watches phase failure will not catch a reversed supply after switchboard work.

Insulation and earth leakage

CM-IWN-AC for electrically isolated AC mains and CM-IWN-DC for isolated DC mains.

These are for IT systems where there is no direct earth reference, so a first earth fault does not trip anything and needs detecting before a second one does.

Motor load monitoring

CM-LWN monitors load states of single and three-phase asynchronous motors.

Useful for detecting an underloaded motor, which an overload relay will never catch. A pump running dry or a conveyor with a broken drive draws less current, not more.

Thermistor motor protection

CM-MSE, CM-MSS and CM-MSN protect motors with integrated PTC resistor sensors.

Thermistor protection measures winding temperature directly where an overload relay infers it from current. On a motor that runs at low speed on a VSD, or one with restricted cooling, the thermistor is the one that actually protects it.

If a motor has PTC sensors wired out to the panel, there should be a CM-MS relay reading them.

Other categories in the range

What to record before replacing a CM relay

These relays are configured on the front, so a replacement is easy if you capture the settings and difficult if you do not.

The full designation including the trailing digits and letter.

Which terminal the measured wire lands on, on the multi-range current relays.

All DIP switch positions. Over or under, open or closed circuit, latching.

All four potentiometer settings. Threshold, hysteresis, tripping delay, start-up delay.

A photograph of the front of the relay covers most of that in one go, and is the single most useful thing to do before removing one.

Open circuit or closed circuit

Worth understanding because it inverts the behaviour.

In the closed circuit principle the output relay is energised in the healthy state and drops out on a fault or loss of supply. That is fail-safe.

In the open circuit principle the relay energises on the fault.

A replacement set to the wrong principle will read as permanently faulted or permanently healthy. It is a DIP switch, and it is the setting most often missed.

What we can supply

Current production. The full CM range across current, voltage, three-phase, insulation, motor load, thermistor, temperature and level monitoring, plus the C5xx panel mounted range. Quoted with a delivered lead time.

Discontinued variants. Earlier CM generations and superseded designations. Surplus and tested used.

Bulk. Monitoring relays are a consumable in a motor control centre. Send us the list.

Related

ABB overview. CP power supplies, which share the 1SVR order number block.

AF contactors and overload relays for motor protection by current, and PSTX soft starters which have protection built in.

For monitoring relays from other brands see Pilz PMD and Phoenix Contact MINI Analog.

Send us your part numbers

The 1SVR order code is definitive. If you only have the front label, send a photo, because that captures the designation and the settings at the same time. Quotes in NZD including GST.

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Industech is an independent supplier of new, surplus and used industrial automation equipment. We are not an authorised distributor of, approved by, sanctioned by, or affiliated with ABB Ltd. ABB is a trademark of ABB Ltd. All trademarks, brand names and order codes are the property of their respective owners and are used here for identification purposes only.