A medium-voltage contactor breaks load and overload currents well, but it is not built to interrupt a short circuit. The fuse does that job. IEC 62271-106 defines the crossover point between the two, the take-over current, and three coordination types according to the damage accepted in the starter.
Short answer
A medium-voltage contactor is meant to switch a motor, not to interrupt a short circuit. IEC 62271-106, 2011 edition, says so in its scope: the contactors and starters it covers generally do not have adequate capacity to interrupt short-circuit currents. That is why a short-circuit protective device, almost always a current-limiting fuse, is associated with them. Coordination is the split of duty between the two: the contactor interrupts up to a certain current and, above it, the fuse does.
The standard applies to indoor contactors and starters in systems above 1,000 V and up to 24,000 V, three-pole, in three-phase systems.
What split of duty the standard requires
Clause 4.107.1 sets the general conditions for coordination:
- The fuse sits on the supply side of the contactor.
- Its breaking capacity cannot be lower than the prospective short-circuit current at the point of installation.
- The overload relay setting must prevent the fuse from operating instead of the contactor on normal service overloads, including the motor locked-rotor current.
- At currents equal to the contactor breaking current in utilization category AC-3, the fuse must withstand those currents at least for the tripping time of the overload relays.
Put simply: on an overload, the contactor opens, ordered by the relay. The fuse must not blow because of an overload the motor can withstand, or because of starting. On a short circuit, the fuse must operate before the contactor is stressed beyond what was accepted.
The standard adds a safety condition: at any overcurrent the starter is suitable for, external manifestations such as flames or hot gases must not exceed a safety perimeter declared by the manufacturer.
The take-over current
If the contactor is tripped by a relay (a “trip-operated” controller), there is a current above which the fuse acts before the contactor. The standard calls it the take-over current and defines it as the current coordinate of the crossover between the relay-initiated opening time of the contactor and the time-current characteristic of the short-circuit protective device (definition 3.7.115).
Two values are distinguished:
- Minimum take-over current. It comes from the crossover with the maximum breaking time of the contactor, plus, where applicable, the maximum time of an external relay, and the minimum pre-arcing time of the fuse.
- Maximum take-over current. It comes from the crossover with the minimum opening time of the contactor and the maximum operating time of the fuse with the highest rated current.
- Does the current exceed the take-over current?
- No: The relay gives the order and the contactor interrupts, with no material damage
- Yes: The fuse limits and interrupts. Which type does the starter declare?
- Type c: Damage only to the main contacts
- Type b: Contacts and arc chambers; the relay may be altered
- Type a: Any damage, except to the outer enclosure
For currents that do not exceed the maximum take-over current, the standard requires that there is no material damage to the contactor or the starter and that they can operate normally afterwards (clause 4.107.3). For higher currents, interruption is left to the fuse and the contactor may be damaged.
The informative annex describes how the practical value is obtained: over the maximum pre-arcing curve of the fuse, a time equal to the minimum opening time of the contactor is superimposed, or the minimum response time if a relay operates it. The associated current is the take-over current, and it must not exceed the rated take-over current, which the fuse manufacturer determines and which is used in the test.
Coordination types by accepted damage
Clause 4.107.3 classifies coordination by the amount of damage accepted above the maximum take-over current. It is verified by test.
| Type | Damage the standard accepts |
|---|---|
| a | Any damage, except external damage to the enclosure. It may require replacing the whole device or key parts, in addition to those of type b. |
| b | The characteristics of the overload relay may be permanently altered. The remaining damage is limited to the main contacts or the arc chambers, which may need replacement or attention. |
| c | Damage is limited to the main contacts of the starter, which may need replacement or have welds to break. |
Cases where the application requires a practically zero risk of contact welding are agreed between manufacturer and user and are not covered by the standard. Note 2 of clause 4.107 adds that the same contactor and fuse combination may meet more than one coordination type for different values of rated short-circuit current.
What each manufacturer declares
Clause 4.107 splits the information as follows:
- Contactor or starter without its own short-circuit protection. The manufacturer states the maximum cut-off current of the intended fuse, the maximum breaking capacity, the short-time current or Joule integral it withstands, and the maximum peak current it withstands.
- Fuse manufacturer. Declares the maximum peak current and the maximum Joule integral the fuse lets through, as a function of the short-circuit current, and the time-current characteristics.
- Starter with built-in fuse. The manufacturer declares the coordinated devices and their characteristics, the damage type, the rated breaking current and the rated making current.
Note 1 explains why the current is called “prospective”: the series impedance of the combination and the fast operation of the fuse reduce the short circuit to a value clearly lower than the one it would reach, and they distort the waveform.
Contactor utilization categories and duty cycles are covered in contactor duty cycles and utilization categories. Switching motors with locked-rotor current is covered in motor switching and locked-rotor current.
How it is verified
Clause 6.106 requires the breaking capacity of the fuse to be verified by the fuse’s own breaking tests, and its resistance to overloads by separate tests. Coordination and damage type are verified with special type tests, in three sequences:
- Sequence A. A 100 % breaking test at the rated short-circuit current of the controller.
- Sequence B. A 100 % making test at that same current.
- Sequence C. Three breaking operations near the take-over point, at a current equal to or greater than the maximum take-over current and not lower than seven times the rated operational current of the controller. In this sequence the fuse is replaced by solid connections.
The standard allows an alternative method for sequences A, B and C. If three make-and-break operations are successfully performed at a current equal to or above the maximum take-over current, sequence C is not required.
What not to do
- Do not replace the fuse with one of another rating or make without reviewing the take-over current. The curves of two manufacturers are not interchangeable.
- Do not set the overload relay above what the fuse withstands: the crossover shifts and the fuse may operate where the contactor was supposed to.
- Do not assume a contactor can break the short circuit of its feeder. Outside the declared coordination, it is not designed for that.
- Do not confuse utilization category AC-3 with coordination: the first describes the load; the second, the split of duty with the fuse.
When to call us
At Tiza Group we verify with you that the fuse, the contactor and the relay of your medium-voltage starter work as the manufacturer declares, before energization. Learn about our motor and starter testing service. Write to us and we will quote it. Field testing is done in Colombia, or elsewhere on request.
Related guides
- MV contactor: duty cycles and utilization categories
- MV motor switching: locked rotor
- Medium-voltage starter: types, protection and tests
- Vacuum medium-voltage contactor: fuses and tests
- Medium-voltage fuse: limiting, expulsion, coordination
References
- IEC 62271-106, High-voltage switchgear and controlgear - Part 106: Alternating current contactors, contactor-based controllers and motor-starters, 2011 edition, clauses 1.1, 3.7.115 to 3.7.117, 4.107 (4.107.1 to 4.107.3), 6.106 (6.106.3.1 to 6.106.3.4) and 8.102.8.1.
Frequently asked questions
What is the take-over current of a motor starter?
It is the current at which the contactor opening time, initiated by the relay, crosses the time-current curve of the fuse. Below that value the contactor interrupts; above it, the fuse takes over the interruption. IEC 62271-106 defines it as the take-over current.
What is the difference between type a, b and c coordination?
The damage the standard accepts after a short circuit changes. In type c the damage is limited to the main contacts. In type b the characteristics of the overload relay may also be altered and the arc chambers may show wear. In type a any damage is allowed, except to the outer enclosure.
Who declares the coordination, the contactor manufacturer or the fuse manufacturer?
Both, each for their own part. The fuse manufacturer declares the time-current curves, the peak current and the Joule integral (I²t) the fuse lets through. The starter manufacturer declares the coordinated devices, the damage type and the rated making and breaking currents.