Relays Selection Guide
Covers residual current, thermal, protection, timer, automation and reactive power control relays; choose by what the relay measures and its function.
This guide explains what to look at when choosing products in the Relays category.
Which relay is in which category
The word relay is used in electrical equipment for very different devices. This category contains only two switchgear protection groups; the others are in their own categories:
- Residual Current Devices: devices that detect current leaking to earth and isolate the line with their own contacts.
- Thermal Overload Relays: overload relays that detect the overload current of a motor and drop out the contactor.
- Protection Relays: electronic relays that monitor phase loss and phase sequence, undervoltage and overvoltage, and overcurrent and undercurrent. Phase and Motor Protection Relays and Voltage Protection Relays are under this group.
- Timer Relays: timing functions such as on-delay, off-delay and star-delta changeover.
- Automation Relays: general-purpose relays with sockets, PLC and PCB relays, and solid state relays (SSR).
- Reactive Power Control Relays: devices that manage the capacitor steps in a power factor correction panel.
The difference between a residual current relay and a thermal overload relay
The two devices measure different quantities. A residual current relay looks at the sum of the currents passing through the phase and neutral conductors; if the sum deviates from zero, part of the current is leaking to earth. Because it is not concerned with the magnitude of the load current, it does not notice an overloaded motor. A thermal overload relay, on the other hand, monitors the thermal effect of the load current in each phase; the milliampere-level current flowing to earth from a housing with damaged insulation is outside its measuring range.
The ways they open the circuit are also different. A residual current relay isolates the line with its own main contacts and is reset manually. A thermal overload relay has no power contact; its auxiliary contact opens the coil circuit of the contactor, and the contactor stops the motor.
The third difference is in the setting. The threshold of a modular residual current relay is fixed and is chosen together with the product. In a thermal overload relay, the current value is set within the setting range to the motor's nameplate current.
Which device against which fault
- Electric shock and earth leakage caused by insulation faults: residual current relay.
- Motor running above its rated current for a long time and jamming: thermal overload relay.
- Phase loss, phase sequence error, voltage drop or rise: protection relays.
- Short circuit: none of these relays is designed to interrupt short-circuit current; this task belongs to fuses and switches.
Brands in this category
- Schneider Electric: Acti9 iID and Easy9 residual current relays; TeSys LRD, TeSys K and EasyPact TVS thermal overload relays.
- ABB: F200 and FH200 residual current protection switches; TF and TA thermal overload relays, EF electronic overload relays.
- Siemens: 5SV residual current protection relays and 3RU2 thermal overcurrent relays.
Chint NL1 residual current relays and Tense TAR residual current detection relays are also listed in the residual current group.
Order notes
Products in both subgroups are sold by the piece. Some products are listed with a lead time; the delivery time is confirmed separately for these products. If you have a manufacturer code such as A9R41463, F204 AC-80/0.3 or 3RU2116-1GB0, just type it into the site search.
Protection and Control Relay Types Comparison Table
| Relay Group | Monitored / Controlled Parameter | Adjustment and Output Form | Typical Application | Category Link |
|---|---|---|---|---|
| Phase Failure & Voltage Monitoring Relay | Phase sequence, phase loss, asymmetry, over/undervoltage | Fixed or adjustable threshold + trip delay · 1 CO / 2 CO | Protecting motors, elevators, compressors and main incoming panels | Voltage Monitoring Relays |
| Timer Relay (On-Delay / Off-Delay / Star-Delta) | 0.1 s – 100 h timing range | On-delay, off-delay, flasher, star-delta transition | Motor starting, lighting automation and sequential machine logic | Timer Relays |
| Thermal Overload Relay | Motor overload current and phase imbalance | Direct mounting beneath contactor · Class 10A/20 | Motor winding overload protection in contactor starter assemblies | Thermal Relays |
| Liquid Level, Photocell & Thermostat Relay | Electrode conductivity, ambient lux, temperature | Adjustable sensitivity · Changeover relay output | Submersible pump dry-run protection and outdoor lighting control | Relays |
Frequently asked questions
Can a thermal overload relay be used instead of a residual current relay?
No. A thermal overload relay responds to the thermal effect of the phase current the motor draws and trips only when the current stays above the setting value for a long time. Earth leakages that are dangerous to people are at the milliampere level, and this current stays far below the setting range of a thermal overload relay. A residual current relay, in turn, does not detect motor overload. For this reason the two protections are not used in place of one another but together, as separate devices.
In which category are digital thermal overload relays listed?
Bimetal thermal overload relays and electronic overload relays such as ABB EF and Schneider Electric LR97D are in the Thermal Overload Relays subcategory. Tense TRM digital thermal overload relays, KON-TER models with a built-in contactor and Entes AKC current relays are listed in the Current Protection Relays category under Protection Relays; the relays in this group can monitor undercurrent as well as overcurrent.
Does a phase protection relay do the same job as a thermal overload relay?
No, one monitors voltage and the other current. A phase protection relay detects phase loss, phase sequence error and voltage imbalance on the mains side, and can also prevent the contactor from pulling in before the motor starts. A thermal overload relay looks at the motor current; it trips on overload and jamming. Phase-loss-sensitive thermal overload relays also protect a motor left on two phases, but they do not detect reversed phase sequence. For this reason the two relays can be used together on the same motor feeder.
Where are a residual current relay and a thermal overload relay connected in a panel?
A residual current relay is fitted on the distribution side at the head of the line or group of lines it will protect, and all phase conductors and the neutral of that line pass through the relay; a single relay can protect several feeders. A thermal overload relay, on the other hand, is separate for each motor: it is connected in series at the contactor output, in the three phases going to the motor, and its auxiliary contact enters the coil circuit of that contactor. The manufacturer's diagram is the basis for connection details.