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 usine chimique et Éclairage à indice de protection contre les explosions sûr... 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.
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Avec plus d'une décennie d'expérience, il est ingénieur électricien explosion-proof chevronné spécialisé dans la conception et la fabrication de produits de sécurité et anti-explosion. Il possède une expertise approfondie dans des domaines clés tels que les systèmes antiprédétection d'explosion, l'éclairage nucléaire, la sécurité maritime, la protection contre les incendies et les systèmes de contrôle intelligents. Chez Warom Technology Incorporated Company, il occupe des postes de direction doubles en tant que Directeur adjoint de l'ingénierie pour les affaires internationales et Chef du département international R&D, où il supervise les initiatives de R&D et assure la livraison précise des documents de conception pour les projets internationaux. Engagé dans l'amélioration de la sécurité industrielle mondiale, il se concentre sur la traduction de technologies complexes en solutions pratiques, aidant les clients à mettre en œuvre des systèmes de contrôle plus sûrs, plus intelligents et plus fiables dans le monde.
Qi Lingyi