1Why DC is harder to break
When contacts carrying current open, the air between them ionizes and an arc forms. The arc is a conductor at a temperature the factory puts at up to 20,000 C, hot enough to erode the contacts and anything near them.
On AC, the current passes through zero twice a cycle. The arc dies at the next zero, within about 8 ms at 60 Hz, and the contactor's job is to keep it from restriking. On DC there is no zero. Once the arc forms it burns for as long as the voltage across the gap can sustain it, which at battery voltage can be indefinitely. The contactor has to put the arc out itself.
Three things put out a DC arc: stretching it until the gap voltage exceeds what the source can drive, splitting it across plates so each short arc needs its own anode and cathode drop, and cooling it. The C303, C310 and C320 do it with permanent magnets. A current-carrying arc sits in the magnet field and the Lorentz force pushes it off the contact tips and up into the arc chamber, where it is stretched and split between insulating plates until it goes out. The chamber is open to air, so the pressure of the arc vents instead of building, and there is no fill gas to lose over the life of the part.
One consequence worth knowing: a single magnet field pushes the arc in one direction, so a contactor built that way breaks well with current flowing one way and poorly the other way. Schaltbau's arc chamber has two sections with the magnets placed so the arc goes up into the left or right section depending on current direction. That is what "bidirectional" means on the datasheet: the same breaking capability for charge and discharge through one device.
The other common architecture seals the contacts in a chamber filled with pressurized gas, such as hydrogen or nitrogen, which cools the arc well in a small package. The trade is that the gas is the quench medium. The factory notes that it can leak away slowly over thermal cycling, and that the loss cannot be detected in normal operation. Both architectures have their place; the honest comparison is on identical conditions, both current directions, electrical life at the same breaking current and end-of-life behavior. Our comparison chart lists the eight points and what to ask each vendor. One point goes to the sealed design. An air-break part derates with altitude: the C310 datasheet limits it to 4,500 m at 1,000 V and 3,500 m at 1,500 V, and the C320 to 5,000 m, with more heating above 2,000 m. The break inside a sealed chamber does not depend on altitude.
2Reading the datasheet
The front of a DC contactor datasheet carries a voltage and a current. Neither is a rating you can design to by itself. Here is what each line on a Schaltbau sheet means, using the C310, C320 and C303 as the examples.
Rated insulation voltage Ui and rated operational voltage Ue. Ui is the voltage the insulation system is built for; Ue is the voltage at which the switching ratings are stated. They move with pollution degree: the C310K is rated 1,000 V at pollution degree 3 and 1,500 V at pollution degree 2, which is a statement about the enclosure it sits in, not only about the part. The smaller-chamber C310A is rated 1,000 V. The C320 is 1,500 V at PD3 with a 1,800 V Ui. Rated impulse withstand voltage Uimp (10 kV on both) is the surge the gap holds off.
Conventional free-air thermal current Ith. The current the part carries continuously without exceeding its temperature limits, in free air, with the stated terminal conductor and ambient. It is a carry rating, not a switching rating. The C310/500 carries 500 A at 85 C ambient with 185 mm2 conductors and a 65 K terminal rise; the C320K/1000 carries 1,200 A at 60 C with 800 mm2 and 1,000 A with 600 mm2. Change the copper, the ambient or the enclosure and the number changes. A Schaltbau engineer, writing in Konstruktionspraxis, notes that in a ventilated cabinet it is not unusual to run above Ith by about a quarter, by agreement with the manufacturer, and that the maximum current and the maximum ambient depend on each other (the white paper carries the derating curve). Ask for that curve at your conditions.
Rated operational current Ie and utilization category. Ie is the current the part is rated to make and break repeatedly, at a stated voltage, for a stated duty. It is far below Ith. The C310 carries 500 A but its DC-1 Ie is 60 A at 750 V under IEC 60947-4-1, and 50 A at 600 V under the UL "DC general use" line. The C320 carries 1,200 A and its DC-1 Ie is 150 A at 1,500 V (IEC) or 80 A at 1,500 V (UL). The electrical endurance figure is stated at its own point. The C310's 6,000 operations are at 750 V and 60 A, its DC-1 rating, at an L/R of 1 ms. The C320's 6,000 operations are at 1,250 V and 120 A, also at 1 ms. A cycle count without its voltage, current and time constant is not a rating.
Making capacity Icm. The current the contacts can close into without welding, at L/R of zero: 2,500 A on the C310 (monostable, horizontal), 3,000 A on the C320, 2,000 A on the C303. This is the inrush question. If the DC link capacitor is charged through the main contactor, this is the number that decides whether the contacts survive the first close. A precharge contactor and resistor (the C303 has a dedicated precharge variant) brings the bus up first so the main contactor closes into almost nothing.
Short-time withstand current Icw. The fault current the closed contacts carry for a stated time without welding, so the fuse has time to clear: 3,000 A for 1 s on the C310, 4,500 A for 100 ms on the C320, 5,000 A for 20 ms on the C303. The white paper's short-time table for the 500 A class runs from 600 A for 600 s to 12,000 A for under 5 ms. Current through the closed contacts also creates a magnetic force that pushes them apart, and that force climbs steeply with fault current. The contact spring force is what holds the part closed through the let-through.
Breaking capacity. The current the part can interrupt at a stated voltage and a stated circuit inductance, with the number of operations it is good for at that point. The C310K table is given at a maximum of 0.25 mH, each point good for ten to twenty-five operations:
| Voltage | One contact | Two contacts in series |
|---|---|---|
| 1,500 V | 300 A | 1,000 A |
| 1,000 V | 500 A | 1,500 A |
| 900 V | 700 A | |
| 750 V | 1,000 A | |
| 500 V | 1,500 A |
The smaller-chamber C310A has its own, lower table. The C320K/1000 is printed at the circuit time constant, thirty operations at each point:
| Voltage | Time constant L/R | Breaking current |
|---|---|---|
| 1,500 V | 0.15 ms | 800 A |
| 1,500 V | 1 ms | 450 A |
| 1,000 V | 0.2 ms | 1,600 A |
| 1,000 V | 1 ms | 1,300 A |
Read the inductance column every time. The energy the arc has to absorb rises with the circuit inductance, and the factory's example is that the C294's breaking capacity rises by 300 percent when the circuit time constant drops from 15 ms to 1 ms. A breaking rating proven in a low-inductance test circuit does not transfer to a long cable run to a battery string.
Pole impedance and power loss. The contact resistance sets the heat. About 120 microohm on the C310, about 50 microohm on the C320 (50 W per pole at 1,000 A), typical 100 microohm on the C303. Loss rises with the square of current, so at 1,000 A the difference between 50 and 150 microohm is 100 W of heat per pole that the enclosure has to shed.
Mechanical endurance. Operations with no current: 2,000,000 on the monostable C310, 1,000,000 on the C320. This is the headline number on most catalogs and it is the one that matters least for an isolator.
7Installation
An open-chamber contactor vents arc plasma through the openings in its arc chamber. The factory manuals ask for three things: the minimum clearances to grounded, magnetic or magnetized parts on all sides and above the chamber (the values are in each catalogue), enough ventilation in the mounting area to carry the plasma away, and a mounting position within the ones the sheet allows (horizontal or vertical, never upside down for the C320; the making capacity of the C310 falls from 2,500 A horizontal to 2,000 A vertical). Switching near the maximum breaking capacity may need larger clearances; ask.
Terminal torque and conductor size come from the sheet: M8 at 10 to 12 Nm and 600 to 800 mm2 on the C320, M6 at 5 to 6 Nm on the C303, M8 at 4.8 to 6 Nm on the C310/150 and M10 at 8 to 10 Nm on the /300 and /500, with the conductor the sheet ties to each current rating, from 50 mm2 up to 2 x 150 mm2. Control wiring differs by part. The C310 coil lands on 6.3 x 0.8 mm flat tabs and its auxiliaries on 2.8 x 0.5 mm tabs. The C320 auxiliaries take 6.3 x 0.8 mm receptacles. The C320 coil takes a screwless terminal, 0.5 to 1.5 mm2 with a ferrule or up to 2.5 mm2 without. The C303 uses a 5-pin push-in connector rated to 1.5 mm2, or 4.8 x 0.5 mm tabs on some versions. The part needs an enclosure. The C310 and C320 sheets give its own ingress rating as IP00.
8Mirror contacts and the safety circuit
A controller that opens a contactor needs to know the contacts actually opened. A welded main contact with a normal auxiliary reporting "open" is the failure a safety circuit exists to catch. IEC 60947-4-1 Annex F defines the mirror contact: an auxiliary NC contact that cannot be closed while the main NO contact is closed. The C303 carries one integrated, the C310 up to two, the C320 up to four auxiliaries of which two can be mirror contacts. The mirror contact is a feedback element for the safety circuit; whether the system meets a performance level is the machine's functional safety assessment, not the contactor's rating.
