An overload relay's trip class is a number, commonly 5, 10, 20, or 30, that describes how long the relay will tolerate a large overcurrent such as locked-rotor current before it trips the motor offline. The number corresponds roughly to the maximum time in seconds the relay allows the motor to draw locked-rotor current before tripping, so a class 10 relay trips relatively quickly and a class 30 relay allows much longer. The trip class is not chosen for the relay's convenience; it is matched to how long the specific motor takes to accelerate its load, so a motor that takes a long time to reach speed gets a higher trip class that will not cut it off mid-start. This page explains how the class sets the trip curve, how to choose one, and how it coordinates with the starter and monitoring.
Overload Trip Class in one line: An overload relay's trip class, such as 5, 10, 20, or 30, defines how long the relay tolerates a large overcurrent, roughly locked-rotor current, before tripping, with the number approximating that maximum time in seconds. A lower class trips faster, protecting the motor sooner, while a higher class allows more time for a motor that accelerates slowly. The class is chosen to exceed the motor's safe acceleration time so the relay does not trip during a normal start, while still protecting the motor within its thermal limit during a genuine stall.
An overload relay protects a motor with an inverse-time characteristic: the further above rated current the motor runs, the faster the relay trips, mimicking the way heat builds in the windings. The trip class fixes where that curve sits in time by defining the maximum time the relay will allow the motor to draw locked-rotor current, taken as a specific multiple of the motor's rated current, before it trips. The class number is essentially that time in seconds, so a class 10 relay will trip within about ten seconds at locked-rotor current, a class 20 within about twenty, and a class 30 within about thirty, with a class 5 tripping fastest of all.
Shifting the trip class does not change the shape of the inverse-time curve so much as slide it along the time axis. A higher class gives the motor more time at every level of overcurrent before tripping, which is what a slow-accelerating motor needs to get through its start, while a lower class trips sooner at every level, giving quicker protection for a motor that comes up to speed fast. Some relays offer intermediate or adjustable classes, and variants such as class 10A exist, but the common menu of 5, 10, 20, and 30 covers most needs.
The reason the class is defined at locked-rotor current is that this is the demanding current a motor draws at the very start and during a stall, and it is precisely the condition where the overload must make its most important decision. During a normal start the motor draws near locked-rotor current only for the brief time it takes to accelerate, so the relay must allow that current for at least that long. During a stall the motor draws locked-rotor current indefinitely, so the relay must trip before the motor's thermal damage limit is reached. The trip class is the setting that positions the relay between these two, and getting it right is what makes the motor both start reliably and stay protected.
The governing principle is that the trip class must exceed the motor's safe acceleration time, the time the motor takes to bring its load up to speed while drawing high starting current. If the class is set below the acceleration time, the relay trips the motor partway through every normal start, because from the relay's point of view a start that lasts longer than its class looks like a sustained overcurrent worth tripping. So a motor and load that accelerate quickly can use a low class such as 10, while a motor and load that take a long time to reach speed need a higher class such as 20 or 30 so the relay rides through the extended start.
Class 10 is the common default for standard motors driving loads that start in a normal, moderate time, and it gives relatively quick protection. Class 20 suits motors with somewhat higher-inertia loads that take longer to accelerate, and class 30 is reserved for genuinely long-accelerating loads, high-inertia machines like large fans and certain pumps, and conveyors, where the motor may draw high current for many seconds as it slowly brings the mass up to speed. Class 5 is used where very fast protection is wanted and the load accelerates almost instantly, or for particularly sensitive motors, and it is less common.
There is a real tension in the choice. A higher class protects the start but delays protection during a genuine fault, so a class 30 relay lets a stalled motor draw locked-rotor current for up to about thirty seconds before tripping, which must still be within the motor's thermal damage limit at that current. This is why the class cannot simply be raised to eliminate nuisance trips; it must stay below the motor's ability to survive locked-rotor current. The correct class is the lowest one that still comfortably exceeds the actual acceleration time, giving the fastest protection that does not nuisance-trip the start, and it is verified against both the load's acceleration time and the motor's thermal damage curve.
The trip class does not stand alone; it coordinates with the rest of the starter and the upstream protection. The overload handles sustained overcurrents like an overloaded or stalled motor, while a separate short-circuit device, a fuse or a breaker, handles the much larger currents of an actual fault. These must be coordinated so the overload rides through the legitimate starting current that its class permits while the short-circuit device stays clear of that starting current too but acts instantly on a true fault. A trip class chosen without regard to the short-circuit protection can leave a gap where a fault is not cleared promptly or an overlap where nuisance trips occur, so the class is set as part of a coordinated protection scheme.
The trip class also has to suit the starting method. A motor started with a reduced-voltage method or a soft starter accelerates differently and draws a different current profile than one started across the line, and a longer, gentler start may need a higher trip class to ride through even though the peak current is lower, because the elevated current lasts longer. Matching the class to both the load's inertia and the way the motor is started ensures the overload protects without tripping the start regardless of which starting scheme is used.
From a field-operations standpoint, a cloud SCADA system that captures motor current turns the trip class from a paper setting into an observable one. Seeing the actual start current profile lets an operator confirm that the real acceleration time fits comfortably within the chosen trip class, so the relay has margin and is not tripping near the edge of the start. If starts begin drifting longer, from an increasing load or a mechanical drag, the current trend shows the acceleration time creeping toward the trip-class limit before nuisance trips actually begin, giving early warning. Across a fleet of remote and unmanned sites, correlating overload trips with the start current signatures helps distinguish a genuine mechanical problem from a trip class set too tight for the load, so the fix, whether adjusting the class or investigating the machine, is the right one.
The trip class number approximates the maximum time in seconds the relay will allow the motor to draw locked-rotor current before tripping. A class 10 relay trips within about ten seconds at locked-rotor current, a class 20 within about twenty, and a class 30 within about thirty, with a class 5 tripping fastest. A lower class gives quicker protection; a higher class allows more time for a motor that accelerates slowly to get through its start.
Choose the lowest class whose time comfortably exceeds the motor's safe acceleration time, so the relay does not trip partway through a normal start but still gives the fastest protection. Standard motors with normal-starting loads typically use class 10, higher-inertia loads use class 20, and genuinely long-accelerating loads like large fans and conveyors use class 30. The chosen class must also stay within the motor's thermal damage limit at locked-rotor current so a stall is cleared before damage occurs.
Because a higher class delays protection during a genuine fault. A class 30 relay lets a stalled motor draw locked-rotor current for up to about thirty seconds before tripping, and that must still be within the motor's thermal damage limit at that current, or the motor overheats before the relay acts. The class is a balance: high enough to ride through the normal start, low enough to protect the motor within its thermal limit, so the correct choice is the lowest class that comfortably exceeds the actual acceleration time.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.