The same In 20 kA and Imax 40 kA in two devices, the CJ-T2-C40 for AC systems and the CJ-T2-DC for DC systems, and what changes between them
A surge protection device is chosen for the system behind it, not for the size of the surge alone. Two models in the C&J range carry the same nominal discharge current of 20 kA and the same maximum discharge current of 40 kA, yet one is built for AC distribution boards and the other for DC circuits.
This article explains what separates the two, which ratings decide the choice, and where each model belongs in an installation.
What a Type II surge protection device does
A Type II device sits at the boundary between LPZ1 or LPZ2 and LPZ3, downstream of the service entrance. It limits the residual overvoltage that reaches distribution boards and equipment after the first stage has taken the main surge.
Both models here are pluggable Type II designs declared to IEC 61643-1 and GB 18802.1. The protection element sits in a removable module, a status window shows green when the device is healthy and red when it has reached the end of its life, and the module can be replaced while the installation is still energised.
Why DC needs its own device
The difference is not a marketing distinction. An AC arc is extinguished at the current zero crossing, which arrives twice every cycle. A DC circuit has no zero crossing, so a disconnector that opens under fault has to break a current that keeps flowing in one direction.
Important: That changes both the interruption design and the thermal duty on the varistor. A device rated for AC service is not a substitute on a DC bus, even where the voltage numbers happen to look compatible.
The declared standards follow the same split. The AC model is declared to IEC 61643-1 and GB 18802.1; the DC model is declared to the same two standards and adds YD/T 1235.1 for telecom DC practice.
CJ-T2-C40, the AC surge protection device
The AC surge protection device CJ-T2-C40 covers single-phase and three-phase boards in one to four poles. The Uc options follow the usual distribution voltages, so a 220 V board and a 380 V board are served from the same series.
Each module has a thermal disconnector and a status window. Green means normal, red means the module has operated and should be replaced; the replacement is made at the module, without removing the base from the rail.
CJ-T2-DC, the DC surge protection device
The DC surge protection device CJ-T2-DC is built in 2P and 3P versions, with maximum continuous operating voltage options from 800 VDC up to 1500 VDC on the 3P version. It is intended for DC power systems from LPZ0B-1 upwards, which covers the DC side of photovoltaic arrays, DC distribution and telecom power.
Two thermal disconnection devices are fitted instead of one, and the module is pluggable. A remote signalling contact is provided as a switching contact, with ratings for both AC and DC control circuits, so the state of the device can be reported to a monitoring system.
A maximum backup fuse of 125 A gL/gG is declared. The upstream protection should not exceed that figure, and where the fault level is higher the coordination has to be checked rather than assumed.
Technical Data Comparison
Key technical data of the CJ-T2-C40 (AC):
| Parameter | Specification |
|---|---|
| Type | Type II surge protective device (Class C), pluggable |
| Standard | IEC 61643-1 |
| Poles | 1P, 2P, 3P, 4P |
| Rated voltage Un | 110 / 220 / 380 V AC, 50/60 Hz |
| Max. continuous operating voltage Uc | 140 V; 275 / 320 / 385 / 420 / 440 V |
| Voltage protection level Up | ≤0.8 kV; ≤1.2 / 1.5 / 1.8 / 2.0 / 2.2 kV |
| Nominal discharge current In (8/20 µs) | 20 kA |
| Maximum discharge current Imax (8/20 µs) | 40 kA |
| Response time | <25 ns |
| N-PE path | Uc 255 V; Up ≤1.0 / ≤1.8 kV; In 5 / 12.5 / 25 kA; Imax 15 / 30 kA; response 100 ns |
| Connection | L/N 10, 16 mm²; PE 10, 25 mm²; fuse or switch 32 A |
| Operating environment | -40 to +85 °C, RH ≤95% at 25 °C |
| Mounting | 35 mm DIN rail; fiberglass reinforced plastic housing |
Note: Data as published in the CJ-T2-C40 catalogue; confirm the voltage version when ordering.
Key technical data of the CJ-T2-DC (DC):
| Parameter | Specification |
|---|---|
| Type | Type II DC surge protective device (Class C), pluggable |
| Standard | IEC 61643-1, GB 18802.1, YD/T 1235.1 |
| Configuration | 2P, 3P |
| Max. continuous operating voltage Uc | 800 / 1000 / 1200 VDC; 1500 VDC (3P) |
| Nominal discharge current In (8/20 µs) | 20 kA |
| Maximum discharge current Imax (8/20 µs) | 40 kA |
| Voltage protection level Up | 2.8 kV / 4.0 kV |
| Response time | ≤25 ns |
| Max. backup fuse | 125 A gL/gG |
| Connection | 1.5-25 mm² solid / 35 mm² flexible |
| Remote signalling contact | Switching contact; 250 V / 0.5 A AC; DC 250 V / 0.1 A, 125 V / 0.2 A, 75 V / 0.5 A |
| Operating temperature | -40 to +80 °C |
| Mounting / enclosure | 35 mm DIN rail; thermoplastic, UL94-V0 |
| Weight | 253 g (2P) / 356 g (3P) |
Note: Data as published in the CJ-T2-DC catalogue. 1500 VDC is available on the 3P version.
How to choose between them
- Start with the system, not the surge: an AC board takes the AC model and a DC bus takes the DC model, whatever the fault level at the point of installation.
- Check Uc against the highest sustained voltage, not the nominal.
- Compare Up with the impulse withstand of the equipment: the residual voltage at the terminals is what the load actually sees.
- Read the backup fuse as a limit: the upstream device protects the SPD as well as the circuit.
- On DC, confirm polarity marking and configuration before ordering.
Where each model fits
- AC distribution boards: household boards, computer rooms and information equipment.
- AC, C grade lightning protection: installed at the joint of LPZ1/LPZ2 and LPZ3.
- DC side of photovoltaic arrays: between the array and the inverter.
- DC distribution and telecom power: DC cabinets and remote sites following YD/T 1235.1.
About C&J Electrical
Zhejiang C&J Electrical Holding Co., Ltd. manufactures low-voltage electrical products, mainly circuit breakers, surge protection devices and related protection products. The company supports OEM brand customization and can ship products as SKD kits for local assembly.
For more information on the CJ-T2-C40, the CJ-T2-DC or to request a quotation, please contact us.
Post time: Nov-04-2026