Explosion proof motor protection relay settings determine whether a motor is isolated before winding temperature reaches an unsafe level or after the damage is already done. Most setting errors we see during factory audits and commissioning work do not come from a missing protection function. They come from applying standard industrial motor practice to motors installed in Zone 1, Zone 2, Division 1 or Division 2 locations. This article explains how to set overload and short circuit pickup, how to coordinate with the contactor and upstream breaker, and which documents to check before the first start. Get the settings right and the relay becomes the last line of defense that actually works.
Why Do Explosion Proof Motor Protection Relays Need Different Settings?
A standard motor protection relay is sized to protect the winding. An explosion proof motor protection relay has to protect the winding inside an ignition risk envelope. If the overload element carries the motor past its temperature class, the enclosure is no longer the issue. The winding surface becomes the ignition source.
The main difference is thermal headroom. A T4 motor can reach a maximum surface temperature of 135 degrees Celsius in normal operation and under fault conditions. At full load, the winding is already close to the permitted limit. If the relay allows a 10 percent overload for long periods because someone used a general purpose setting, the motor surface temperature rises and the area classification is no longer respected. We have seen this in chemical and pharmaceutical plants where motors were set from a busbar schedule that had been prepared for a nonhazardous utility block.
For hazardous area motors, the relay setting begins with the nameplate full load current, not with a maintenance estimate. It also has to account for high ambient temperature, reduced ventilation, frequent starting, and the risk of locked rotor inside a vessel or mixer. None of those conditions make the setting more generous. They make the setting tighter.
How Should Overload Pickup Be Set from the Motor Nameplate?
The overload pickup should be set from the motor nameplate full load current at the actual operating voltage. If the relay is fed by current transformers, convert the primary pickup to secondary current before entering the value. A 100/1 CT with a motor full load current of 47 amps gives a secondary pickup of 0.47 amps, but many site errors happen when the CT ratio is entered as 100 rather than 100/1 or when the wrong CT tap is selected.
Do not add a service factor unless the motor data explicitly permits it. Many explosion proof motors have a service factor of 1.0 even when the same frame size in a safe area has 1.15. In our project reviews, the fastest way to create a temperature class problem is to set the relay at 1.15 times nameplate current on a motor rated for 1.0 service factor. That extra margin may look conservative, but it is the opposite.
What if the motor nameplate gives service factor above 1.0?
Confirm that the temperature class was certified with the service factor applied. Some North American explosion proof motors carry a 1.15 service factor, but the temperature class is not automatically valid at that overload. The relay should still be set at nameplate full load current unless the manufacturer’s certified nameplate expressly states otherwise. When the data sheet is silent, treat the service factor as zero additional relay margin.
What if the current transformers are not perfectly matched?
Use the actual CT ratio and burden. Protection class CTs are preferred for short circuit detection, but metering class CTs are still common in older plants. If the CT saturates, the relay cannot see the true short circuit current. That is not a reason to raise the pickup. It is a reason to replace the CT or use a relay with lower burden input.
| Setting point | What to check | Common error in hazardous areas |
|---|---|---|
| Overload pickup | Set from nameplate full load current at operating voltage | Applying a safe area service factor of 1.15 to an Ex motor |
| Overload class | Match motor locked rotor withstand time | Class 20 or 30 chosen only to avoid trip on long start |
| Short circuit pickup | Keep above starting current asymmetry and below cable damage | Raised to 15 times FLA to stop nuisance trips |
| Ground fault pickup | Base on system grounding and CT residual current | Isolated panels with ground fault left disabled |
| Phase unbalance | Set 5 to 10 percent negative sequence | Set so high that single phasing is ignored |
| Thermal memory | Confirm retained after relay power loss | Disabled after vendor testing |
For most direct online explosion proof motors, overload class 10 or 20 is appropriate. Class 30 should be used only when the motor starting time and locked rotor withstand data show that Class 20 would trip before the rotor reaches speed. That condition exists on some high inertia fans, centrifuges, and conveyors, but it is a calculation result, not a standard preference.
How Should Short Circuit Pickup Be Set Without Tripping on Inrush?
Short circuit pickup on a motor protection relay is not a backup for the wrong cable size. It is the fast element that clears a bolted fault before the contactor tries to interrupt current beyond its breaking capacity. The pickup must sit above the motor starting current envelope and below the cable and starter damage curve.
For low voltage motors, the instantaneous pickup often falls between 8 and 12 times full load current when the motor is started direct online. The exact value comes from the motor starting curve, the expected asymmetry, the CT performance, and the contactor duty. A setting below the starting current causes a trip on every direct start. A setting far above it, such as 15 or 20 times, can leave a developing fault uncleared long enough for winding damage.
The same calculation applies to short time protection. If the relay has a definite time element, set the delay short enough to protect the contactor and cable but long enough to ride through a normal start. In many medium voltage motor circuits, the short time element is set around 0.1 to 0.3 seconds with a pickup just above the locked rotor current. The precise value depends on the cable damage curve and upstream breaker coordination, not on a standard table.
How should ground fault pickup be handled on a resistance grounded system?
Low resistance grounded systems limit earth fault current, so the ground fault element can be set low without tripping on charging current. A pickup of 10 to 40 percent of the CT ratio is common, but the actual setting must be checked against the system grounding resistor rating. For solidly grounded low voltage systems, use a residual connection or core balance CT and set the pickup low enough to detect a winding fault before it develops into a phase fault.
If your motor is part of a compressor, conveyor, or pump train in a Zone 1 or Division 1 area, the short circuit pickup cannot be chosen without the starting current envelope and the feeder cable damage curve. Send the motor starting curve, locked rotor withstand time, CT ratio, cable size and upstream breaker data to gm*@***om.com, and we will check the setting window before the relay is configured.
Motors on coal conveyors are often started across the line with high inertia and long runback times, which makes the short circuit pickup margin harder to calculate. <Explosion Proof Solutions for Coal Conveyor Safety> covers how protection and control choices affect conveyor stopping and emergency isolation in hazardous areas.
What Setting Errors Do We Find Most Often in Hazardous Area Motor Feeders?
The most common error we find during commissioning is a relay file copied from another motor. The voltage is different, the CT ratio is different, and the overload class is different, but the settings all look acceptable on a screen. The second error is disabling the unbalance element because the plant had a nuisance trip. For an explosion proof motor in a dust or gas zone, unbalanced current on a single phasing condition can quickly raise rotor temperature beyond the certified limit.
We also see short circuit pickup set so high that the upstream breaker operates before the relay. This defeats the point of having a dedicated motor protection relay. In a Flameproof Ex d installation, the contactor and relay may sit inside the same flameproof enclosure, but they are not interchangeable devices. The relay detects, the contactor interrupts, and the breaker backs up. If the setting hierarchy is wrong, the cheapest component becomes the weakest point.
Phase current transformers installed on the wrong phases create another hidden failure. The relay may read a perfectly balanced current from the secondary wiring while the actual motor sees reversed phase rotation or a lost phase. Checking the primary and secondary currents with a clip on meter before the first start catches this. It also gives the commissioning team a record that the relay sees the same current as the motor.
When we review supplier documentation for hazardous area equipment, the relay settings are only as reliable as the certificate behind the product. <Evaluating Weatherproof Electrical Suppliers in China for Industrial Safety> covers how to verify test reports and certification claims before accepting a setting file.
Why Get a Second Check of Motor Protection Relay Settings Before Energizing?
Motor protection settings carry the same ignition risk as the enclosure. Most of the problems we find during audits come from settings that were copied from a safe area motor and never checked against the hazardous area motor thermal curve. A second check before energizing forces someone to prove that the overload pickup matches the nameplate, the short circuit element matches the starting curve, and the CT ratios match the installed equipment.
If the relay is already selected, send the motor nameplate, starting curve, CT ratio, cable schedule and upstream breaker data to gm*@***om.com or call +86 21 39977076 (+86 21 39972657). We will review the protection setting sheet against the motor thermal limit and the area temperature class before the first start.
What Else Should Procurement Teams Ask About Explosion Proof Motor Protection Relays?
How long can an explosion proof motor run on overload before the relay trips?
That depends on the overload class. Class 10 trips in about 10 seconds at 7.2 times full load current from a cold start. Class 20 takes about 20 seconds, and Class 30 about 30 seconds. The relay should trip before the winding reaches the locked rotor thermal limit, not at some fixed time that feels convenient. If the motor data shows a locked rotor withstand time of 12 seconds, Class 30 is wrong regardless of how long the process takes to start.
Does the service factor setting matter if the relay has thermal modelling?
Yes, and this is a common misconception. A thermal model does not remove the need to enter the correct full load current. If the model is fed a current that is 1.15 times too high, it will track a hotter rotor than reality during heavy loading and may allow a lower actual overload to persist. The model is only as accurate as the primary current value and CT ratio entered into it.
Can one relay protect both overload and short circuit in an Ex d installation?
A motor protection relay can provide both functions, but the short circuit element still depends on a breaker or current limiting fuse to clear high fault current. The relay sends the trip command. The power device clears the fault. In an Ex d enclosure, that division of duty has to be confirmed by the short circuit coordination study. The relay cannot replace the breaking capacity of the upstream protective device.
We have a motor in a Division 2 area and the local contractor says Class 20 is fine. How do we know?
In programs we have supported, the correct class is never a policy decision. It comes from the motor starting time and the locked rotor withstand curve. If the motor reaches speed in 4 seconds and the withstand limit is 10 seconds at locked rotor current, Class 20 may still be acceptable but Class 10 offers closer thermal protection. If the start takes 9 seconds, Class 10 will trip before the motor reaches speed. Ask the contractor to show the starting curve and the damage curve, not just the class number. If the curves are not available, share the motor nameplate and duty description with us and we will confirm the recommended setting before the relay is programmed.
If you’re interested, check out these related articles:
ACHEMA 2024 is Underway
Weatherproof IP66 Electrical Enclosures: Industrial Safety Imperatives
Hannover Messe 2024
Explosion Proof Wiring: Essential Standards for Industrial Safety
With over a decade of experience, he is a seasoned Explosion-Proof Electrical Engineer specializing in the design and manufacture of safety and explosion-proof products. He possesses in-depth expertise across key areas including explosion-proof systems, nuclear power lighting, marine safety, fire protection, and intelligent control systems. At Warom Technology Incorporated Company, he holds dual leadership roles as Deputy Chief Engineer for International Business and Head of the International R&D Department, where he oversees R&D initiatives and ensures the precise delivery of design documentation for international projects. Committed to advancing global industrial safety, he focuses on translating complex technologies into practical solutions, helping clients implement safer, smarter, and more reliable control systems worldwide.
Qi Lingyi