Three model prefixes dominate medium-voltage motor switching: CKG3, CKJ5 and JCZ5. Buyers often treat them as interchangeable 12 kV contactors and then discover, at the drawing approval stage, that one of them is a 1,140 V device, another has no short-circuit breaking rating, and the third needs a control supply the panel does not have. The three families do different jobs, and the split is decided by system voltage and switching duty long before price.
This guide compares the three series against the published ratings, then reduces the choice to four decisions: voltage class, duty, short-circuit protection and mounting. It closes with the specification fields that keep a wrong order off the factory floor. If you need the operating principle first, the vacuum contactor overview explains arcing in vacuum and why a contactor is built for frequent operation rather than fault clearing. Browsing the full vacuum contactor range first also helps, because the series names map directly to voltage classes.
If you only need the shortest possible version:
That single difference in fault behaviour — carrying versus breaking — is what most selection errors come down to. A contactor that can only make and break load current must sit behind a fuse or a breaker. A contactor with a published breaking capacity can clear a limited fault itself, which changes the whole protection scheme.
Voltage class eliminates options faster than any other criterion. The published ranges separate cleanly:
| Series | Rated voltage | Rated current | Typical system | Typical use |
|---|---|---|---|---|
| CKJ5 | 380 / 660 / 1,140 V AC | 250 / 400 / 600 A | 400 V, 690 V, 1,140 V | LV motor starters, flameproof starters, remote switching |
| CKJ20 | 2 kV class | 250 / 400 / 630 / 800 A | 1.5 kV to 2 kV | Heavy LV and 2 kV motor and capacitor switching |
| CKG3 | 7.2 / 12 kV | 160 / 250 / 400 / 630 A | 6.6 kV, 10 kV, 11 kV | MV motor starting, transformer and capacitor switching |
| JCZ5 | 7.2 / 12 kV | 250 / 400 / 630 A | 6.6 kV, 10 kV, 11 kV | Repeated MV switching with limited fault breaking |
The practical rule is insulation coordination, not marketing. A 12 kV contactor carries a 42 kV power-frequency withstand and a 75 kV lightning impulse withstand, which sets its pole spacing, its creepage path and its cubicle footprint. Fitting a 1,140 V CKJ5 into a 10 kV circuit is not a rating question that a purchase order can settle — the device has no impulse rating for that insulation level at all.
Conversely, ordering a 12 kV CKG3 for a 690 V motor starter wastes money and space, and often forces a larger cable chamber than the panel has. Match the contactor’s rated voltage to the system’s highest operating voltage, with the standard one-step margin used in the switchgear: 7.2 kV units on 6.6 kV systems, 12 kV units on 10 kV and 11 kV systems, and 1.14 kV units on 690 V systems.
Read the ratings tables carefully and a second split appears. The CKG3 publishes a rated breaking capacity of 8 Ie with 25 operations, a rated closing capacity of 10 Ie with 100 operations, a rated short-time withstand current of 10 Ie (1.6 kA to 6.3 kA across the current range) and a rated peak withstand of 25 Ie (4 kA to 16 kA). Those are load-breaking and fault-carrying numbers, not fault-clearing numbers.
The JCZ5 publishes a rated maximum breaking current of 2,000 A, 3,200 A and 5,040 A for the 250 A, 400 A and 630 A variants, alongside a 4-second thermal stability current of 2,500 A, 4,000 A and 6,300 A, and a rated closed current of the same values. In other words, the JCZ5 is documented to break current above its nominal rating for a limited number of operations.
That distinction drives three common configurations:
If your specification says “contactor shall interrupt fault current”, confirm the number before the order, because a CKG3 in that role needs an upstream protective device, and the switchgear drawing changes with it. The contactor versus breaker comparison covers this boundary in more detail, including why a contactor’s electrical life is quoted in hundreds of thousands of operations while a breaker’s is quoted in thousands.
Utilization category decides the current rating you actually need. AC-3 covers squirrel-cage motor starting with the motor running up to speed before the contactor opens. AC-4 covers plugging, inching and reversing, where the contactor opens starting current repeatedly — a far harsher duty.
The published lives show the penalty clearly. The CKG3 is rated for 300 operations per hour on AC-3 and 120 operations per hour on AC-4, with 60 × 10,000 AC-3 electrical operations and 300 × 10,000 mechanical operations. The JCZ5 publishes 25 × 10,000 electrical operations on AC-3 and 10 × 10,000 on AC-4. The CKJ5 publishes 60 × 10,000 AC-3 operations and 300 × 10,000 mechanical operations, with 300 operations per hour on AC-3 and 120 on AC-4.
For a crane hoist, a winder or a mill duty drive that reverses dozens of times an hour, AC-3 numbers are meaningless — size on AC-4 and expect roughly a quarter of the electrical life. For a pump or fan that starts twice a day, AC-3 life is generous and the cost of oversizing for AC-4 is wasted.
The control circuit is where panel builders get caught. The CKJ5 offers AC 50 Hz control voltages of 36 V, 127 V, 220 V and 380 V, with 3 NO plus 3 NC auxiliary contacts. The JCZ5 publishes AC 110 V and 220 V control, a closing-coil current up to DC 6 A or 3 A, a holding-coil current of DC 0.32 A or 0.16 A and a shunt-trip coil current of DC 2.5 A or 1.3 A. A JCZ5 that needs a DC holding supply will not run from a plain 220 V AC control bus without a rectifier module.
Mounting geometry follows the same pattern. CKJ5 horizontal units are built to sit in a starter chassis; the vertical GKJ5 arrangement exists for deep cubicles. CKG3 units mount in fixed MV cubicles and in mining and hoist control panels, where 3-pole, 4-pole and 5-pole variants with mechanical interlocking are used for reversing duty. JCZ5 units are offered in fixed and withdrawable formats for metal-clad switchgear.
| Parameter | CKJ5 (horizontal) | CKG3 12 kV | JCZ5 7.2 / 12 kV |
|---|---|---|---|
| Rated voltage | 380 / 660 / 1,140 V AC | 12 kV | 7.2 / 12 kV |
| Rated current | 250 / 400 / 600 A | 160 / 250 / 400 / 630 A | 250 / 400 / 630 A |
| Power-frequency withstand | Not published at MV level | 42 kV | 42 kV |
| Lightning impulse withstand | Not published at MV level | 75 kV | Not stated in the published table |
| Making capacity | 10 Ie | 10 Ie, 100 operations | Rated closed current 2,500 / 4,000 / 6,300 A |
| Breaking capacity | 8 Ie; limited breaking 4,500 A (250/400 A), 6,000 A (600 A) | 8 Ie, 25 operations | 2,000 / 3,200 / 5,040 A |
| Short-time withstand | Not published | 10 Ie, 1.6 to 6.3 kA | 4 s thermal stability 2,500 / 4,000 / 6,300 A |
| Peak withstand | Not published | 25 Ie, 4 to 16 kA | Not stated in the published table |
| Electrical life, AC-3 | 60 × 10,000 operations | Published per variant | 25 × 10,000 operations |
| Electrical life, AC-4 | Published per variant | Published per variant | 10 × 10,000 operations |
| Mechanical life | 300 × 10,000 operations | Published per variant | Published per variant |
| Control supply | AC 36 / 127 / 220 / 380 V | AC control, per order | AC 110 / 220 V with DC coils |
| Contact gap | Per variant | Per variant | 1.5 ± 0.5 mm |
| Operating temperature | Per published data | -10 °C to 40 °C | -5 °C to 40 °C |
| Altitude limit | Per published data | 1,000 m standard, 4,000 m plateau type | 1,000 m |
Read the table by column, not by row. Column one describes a low-voltage device. Columns two and three describe medium-voltage devices that differ mainly in how much fault current they are documented to break.
The CKJ5 belongs in LV motor control centres, flameproof starters for hazardous areas, and any application where the switching point is far from the operator. Its 1,140 V rating covers 690 V systems, and the 4,500 A to 6,000 A limited breaking capacity is adequate for LV fault levels when backed by fuses. Where currents climb past 600 A on a 2 kV system, the larger CKJ20 2 kV contactor extends the same principle to 800 A with 60 × 10,000 AC-3 operations and a 2.5 mm contact gap.
The CKG3 12 kV contactor is the default choice for medium-voltage motor starters, transformer feeders and capacitor banks where switching frequency is high and fault clearing is delegated to a fuse or breaker. It is also the series used in mining hoist and winder control, where 4-pole and 5-pole variants with mechanical interlocking reverse a motor. The 160 A variant suits small motors and capacitor steps; the 630 A variant covers large fans, compressors and pumps.
The JCZ5 vacuum contactor fits applications that need repeated operation plus a documented breaking capability: capacitor switching with inrush control, arc furnace and induction furnace supplies, and switchgear where the contactor is the primary switching device and the upstream protection is set above the contactor’s breaking rating. Its three-phase opening synchronism of 2 ms or better matters for capacitor and filter duty.
This is the comparison that generates the most support questions. A buyer searching for a 10 kV contactor will find both families quoted, because both appear in the same catalogue. The CKJ5 cannot be applied at 10 kV at all. If the system is 10 kV or 11 kV, the choice is between CKG3 and JCZ5, and the deciding question is whether the contactor must break fault current or only carry it.
| Mistake | What happens | Correct approach |
|---|---|---|
| Quoting CKJ5 for a 10 kV feeder | Device has no 12 kV insulation level; drawing rejected at approval | Use CKG3 or JCZ5 for 7.2 / 12 kV systems |
| Specifying a contactor to clear fault current | No published breaking rating at the fault level; protection coordination fails | Add a fuse or upstream breaker, or select JCZ5 with a verified breaking rating |
| Sizing on AC-3 for a reversing drive | Electrical life collapses to a quarter of the quoted figure | Size on AC-4 and check operations per hour |
| Assuming an AC control supply | JCZ5 closing and holding coils are DC; the panel has no rectifier | Confirm coil type, voltage and current draw before ordering |
| Ignoring altitude | Standard CKG3 is limited to 1,000 m; insulation derates above that | Order the plateau type for sites up to 4,000 m |
| Mixing 3-pole and 4-pole variants in a reversing scheme | Mechanical interlocking cannot be fitted, or the neutral is left unswitched | State pole count and interlock requirement explicitly |
A contactor quotation is only as good as the data behind it. Include the following and most technical back-and-forth disappears:
The three families are not competitors on price. CKJ5 is a low-voltage device, CKG3 is a medium-voltage workhorse built for frequent load switching, and JCZ5 is the medium-voltage option with a documented fault-breaking capability. Choose the voltage class first, then decide what the contactor has to break, then size for the real duty cycle, and only then compare quotations. A contactor that fits the voltage and the duty but not the protection scheme is the most expensive kind of saving.
If you are still weighing a contactor against a breaker for a specific feeder, the CKJ5 horizontal contactor and the CKG3 pages both list the full published ratings so you can compare them against your fault study directly.
No. The CKJ5 horizontal vacuum contactor is published for 380 V, 660 V and 1,140 V AC main circuits. Its insulation level is a low-voltage level, so it cannot be applied to 6.6 kV, 10 kV or 11 kV systems. For those systems the correct families are CKG3 and JCZ5 at 7.2 kV or 12 kV.
Both are 7.2 kV and 12 kV indoor vacuum contactors with similar current ranges. The CKG3 publishes load-breaking and fault-carrying ratings — 8 Ie breaking, 10 Ie short-time withstand and 25 Ie peak withstand — and is designed for frequent motor, transformer and capacitor switching. The JCZ5 additionally publishes a maximum breaking current above its nominal rating, so it can interrupt limited fault current within that figure.
Only when its published breaking capacity exceeds the calculated fault level and the protection scheme is designed for it. In most medium-voltage motor feeders the contactor is paired with a fuse or an upstream breaker that clears faults, while the contactor handles the frequent switching that would wear out a breaker.
The JCZ5 mechanism uses a closing coil drawing up to DC 6 A or 3 A, a holding coil at DC 0.32 A or 0.16 A and an optional shunt-trip coil at DC 2.5 A or 1.3 A, with AC 110 V or 220 V control-circuit input. The panel therefore needs a rectifier or a DC control bus; a plain AC bus without conversion will not hold the contactor closed.
Use AC-3 when the motor accelerates to full speed before the contactor opens — pumps, fans, compressors. Use AC-4 when the contactor opens starting current — plugging, inching, reversing, crane and hoist duty. Published AC-4 electrical life is roughly a quarter of the AC-3 figure, so the same contactor carries a much smaller load in reversing service.
Yes. The published CKJ5 data carries a voltage condition on the 600 A rating, so the configuration should be confirmed against the actual system voltage before ordering. The 250 A and 400 A ratings are published without that caveat.
