Motor protection circuit breaker setting: how to find the right value

The pump in the cooling circuit has already shut down three times this week. Each time the motor protection circuit breaker tripped. On the third occasion, someone turns the setting dial a little further to the right. After that, all is quiet – until the winding burns out a few months later.

This scenario is typical. A wrongly set motor protection circuit breaker either fails to protect or trips constantly. Both cost time and money. Yet in most cases the right value is quickly found: it is on the motor nameplate.

This article shows step by step how to set a motor protection circuit breaker, which special cases exist and what to do if it keeps tripping despite the correct setting.

Important note first

Setting, testing and replacing motor protection circuit breakers is work on electrical installations. It may only be carried out by qualified electricians or under their supervision. The basis is the German accident prevention regulation for electrical installations (DGUV Vorschrift 3) and the standard for operating electrical installations (DIN VDE 0105-100). The information in the data sheets of the breaker and motor manufacturers always takes precedence over the general guidance in this article.

How a motor protection circuit breaker works

A motor protection circuit breaker (MPCB) combines two protective functions in one device:

• Overload protection: a thermally delayed release – traditionally a bimetal strip, or electronics in newer devices – replicates the heating of the motor. If too much current flows for too long, it trips. This release is adjustable – that is the setting dial on the front.
• Short-circuit protection: an electromagnetic release reacts without delay to very high currents. On most devices its response value is fixed and is a multiple of the upper setting value. You will find the exact value in the data sheet.

Many devices also detect phase loss and then trip earlier, because a motor running on two phases overheats quickly. The motor protection circuit breaker also serves as the main switch for the outgoing feeder: it can be switched on and off by hand and indicates a trip.

The relevant product standards are EN 60947-2 for circuit breakers and EN 60947-4-1 for motor starters and overload protection.

The basic rule: setting value equals rated motor current

Normally you set the motor protection circuit breaker to the rated current of the motor. This value is on the nameplate and applies to the voltage and connection with which the motor is actually operated.

The motor protection circuit breaker must be selected so that this current lies within its setting range, ideally towards the middle rather than at the very edge. A device with a range of 9–12.5 A, for example, is a good match for a motor rated at 11 A.

Step by step: finding the right value

Step 1: Isolate and lock out the installation

Isolate the outgoing feeder, secure it against being switched back on and verify that it is dead. On many devices the setting dial can be adjusted during operation, but checking the wiring and terminals cannot.

Step 2: Read the motor nameplate

The nameplate usually shows two voltages and two currents, for example:

400/690 V Δ/Y – 14.5/8.4 A – 7.5 kW – 50 Hz

This means: at 400 V the motor is operated in delta (Δ) and then draws 14.5 A. At 690 V it is operated in star (Y) and draws 8.4 A. For a motor marked 230/400 V Δ/Y, the star connection – and thus the lower current – applies on the usual 400 V supply.

Step 3: Check the connection in the terminal box

Check how the links in the terminal box are fitted. Three vertical links mean delta; one link across three terminals means star. Only then do you know for certain which current on the nameplate applies.

Step 4: Turn the setting dial to the rated current

Set the value from step 2, in the example 14.5 A. If the value lies between two markings, set it as precisely as possible in between. Do not round up generously.

Step 5: Test the function

Many devices have a way of testing the trip, such as a button or a slot for a screwdriver. Use it to check that the breaker trips mechanically and can be switched back on.

Step 6: Measure the current in operation

After switching on, measure all three phases with a clamp meter. Under normal load, the current should be below the rated current and similar in all phases. Clear differences between the phases point to a problem in the supply, at the terminals or in the winding.

Step 7: Document the setting

Note the set value in the circuit diagram or on a label on the device. That way the next colleague can see immediately whether someone has changed it afterwards.

Special cases you should know about

Star-delta starting

Here it depends on where the overload protection is located. If the motor protection circuit breaker sits in the supply line upstream of the star-delta combination, the full line current flows through it: the setting value is the rated current. If an overload relay sits in the phase lines downstream of the main contactor, only the phase current flows there: the setting value is then 0.58 times the rated current, i.e. the rated current divided by √3.

Fictitious example calculation: motor with 14.5 A in delta. Overload relay in the phase line: 14.5 A × 0.58 ≈ 8.4 A.

Motor on a frequency inverter

A motor protection circuit breaker upstream of a frequency inverter (VFD) protects the supply line and the inverter, but not the motor reliably. The current on the supply side does not correspond to the motor current. In this case, motor protection is provided by the inverter itself, via its current limit, a thermal motor model or a PTC thermistor in the winding. Follow the inverter manufacturer’s specifications here.

Heavy starting

Fans with large flywheel masses, centrifuges or mills take a long time to reach their speed. A motor protection circuit breaker with trip class CLASS 10 can trip in the process, even though the motor is fine. The trip class according to EN 60947-4-1 indicates the maximum time within which the protection trips from cold at 7.2 times the setting current:

Trip class	Tripping time at 7.2 × setting current	Typical use
CLASS 10A	2 to 10 s	light starting
CLASS 10	4 to 10 s	normal starting, the most common case
CLASS 20	6 to 20 s	heavy starting
CLASS 30	9 to 30 s	very heavy starting

The solution is then a device with a suitable trip class – not a higher current setting. But check whether the motor itself is designed for the long run-up.

Single-phase motors and DC

Three-pole motor protection circuit breakers can often also protect single-phase loads. To do this, all three current paths are connected in series as specified by the manufacturer, so that phase-loss detection does not respond. Whether the device is approved for this, and for DC, is stated in the data sheet.

It keeps tripping – what now?

If a correctly set motor protection circuit breaker trips repeatedly, it is doing exactly its job. It is reporting a problem. Look for the cause instead of turning the value up:

• Mechanical overload: stiff bearings, blocked pump, clogged filter, belt tensioned too tightly
• Phase loss or imbalance: loose terminal, blown fuse, burnt contactor contact
• Ambient temperature too high: overheated control cabinet, faulty fan, devices mounted too close together
• Switching too often: every start heats up the motor and the bimetal. Many starts in quick succession lead to tripping.
• Winding damage: an inter-turn fault increases the current. An insulation resistance test with the motor disconnected provides clarity.
• Ageing device: motor protection circuit breakers wear out too. A device that switches off unusually early after many short-circuit trips should be replaced.

Incidentally, creeping bearing damage is an often underestimated cause of rising current draw. Find out more in the article 7 signs of bearing failure.

Typical mistakes when setting

1. Turning the value up after a trip: the motor is then unprotected. An overload destroys the winding without the breaker intervening.
2. Wrong column on the nameplate: with Δ/Y motors, the current for the wrong connection is read off.
3. Catalogue value instead of nameplate: motors of the same power have different currents depending on manufacturer, number of poles and efficiency class.
4. Device at the end of its range: a motor protection circuit breaker set to its upper limit has usually been chosen too small.
5. No documentation: later on, nobody knows which value was correct.

What to specify when ordering a replacement

If a motor protection circuit breaker needs to be replaced, these details ensure a suitable device:

• manufacturer and complete order number of the old device
• setting range, for example 10–16 A
• short-circuit breaking capacity, if stated
• trip class, if not CLASS 10
• add-on parts: auxiliary contacts, signalling contact, undervoltage release or shunt trip, door-coupling rotary handle
• frame size and mounting, connection technology
• motor nameplate data: power, voltage, current, connection

Many outgoing feeders consist of a motor protection circuit breaker and a contactor. What distinguishes a contactor from a relay and what to look out for is explained in our article contactor vs relay.

How we can help

Older switchgear often contains motor protection circuit breakers from a range that the manufacturer no longer offers. It then becomes important to find a device with the same setting range, a suitable frame size and compatible accessories. KTB Europe is a family business from Hamburg and has been sourcing spare parts of all brands worldwide since 1976. Send us the order number, a photo of the nameplate or a description. If the original has been discontinued, we look for a technically suitable alternative.

Find out more on our pages on MRO procurement and discontinued parts. You can send your enquiry via the contact page.

Conclusion

Setting a motor protection circuit breaker correctly is no great art: read the rated current from the nameplate, check the connection, set the value, measure again in operation and document it. Special cases such as star-delta starting, frequency inverters or heavy starting need a little more attention. And if the breaker trips repeatedly, that is a sign of a problem – not a reason to turn the value up.