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The definition of weak current cable and examples of its selection
Wire and cable are materials used in power, communication, and related transmission applications. While there are no strict boundaries between "wires" and "cables," generally, a product with fewer cores, smaller diameter, and simpler structure is called a wire. A non-insulated conductor is referred to as a bare wire, while the rest are considered cables. Conductors with a cross-sectional area greater than 6 square millimeters are classified as large wires, whereas those less than or equal to 6 square millimeters are called small wires. Insulated wires are also commonly referred to as wires. Wire and cable include bare wires, electromagnetic wires, insulated wires for electrical appliances, power cables, communication cables, and optical fibers.
The naming of weak current cables follows specific principles. The product name typically includes: (1) the application or size class, (2) the structural material or type, and (3) any important or additional features. These elements are usually listed in that order, though sometimes features are emphasized by placing them before the structural description. The structure is described from the inside out: conductor → insulation → inner sheath → outer sheath. Some structural descriptions may be simplified or abbreviated without causing confusion.
Examples of cable designations include:
- **SYV 75-5-1**: S = RF, Y = Polyethylene insulation, V = PVC sheath, 75 = 75 ohms, 5 = 5mm diameter, 1 = single core.
- **SYWV 75-5-1**: S = RF, Y = Polyethylene insulation, W = Foamed insulation, V = PVC sheath, 75 = 75 ohms, 5 = 5mm outer diameter, 1 = single core.
- **RVVP 2*32/0.2**: R = Flexible wire, VV = Double insulated, P = Shielded, 2 = 2-core, 32 = 32 copper wires per core, 0.2 = 0.2mm diameter per wire.
- **ZR-RVS 2*24/0.12**: ZR = Flame retardant, R = Flexible wire, S = Twisted pair, 2 = 2-core, 24 = 24 copper wires per core, 0.12 = 0.12mm diameter per wire.
A weak electric cable typically consists of an inner conductor, insulation, outer conductor, and sheath. The inner conductor affects signal transmission due to its resistance. The insulation influences properties like attenuation and return loss. The outer conductor serves as a return path and provides shielding.
Cable codes have specific meanings:
- **RVVP**: Flexible, PVC-insulated and sheathed cable, used for instruments, intercoms, and control systems.
- **KVVP**: Braided shielded, PVC-sheathed cable, used for signal transmission and control in electrical and instrumentation systems.
- **SYWV**: Coaxial cable used for TV and broadband networks, featuring a copper conductor, foamed polyethylene insulation, and a PVC jacket.
- **RVV**: PVC-insulated flexible cable, suitable for household appliances and small tools.
- **AVVR**: PVC-sheathed flexible cable.
- **RV** and **RVP**: PVC-insulated cables.
- **RVS** and **RVB**: Suitable for household appliances and lighting connections.
- **BV** and **BVR**: PVC-insulated cables, used for fixed wiring in electrical equipment.
When selecting weak current cables, several factors must be considered. For example, if the distance between a video camera and the monitor is ≤200 meters, SYV75-3 is recommended. If it's over 200 meters, SYV75-5 is more appropriate. For PTZ control lines within 100 meters, RVV6×0.5 is suitable, while longer distances require RVV6×0.75. Lens control lines use RVV4×0.5, and decoder communication lines need RVV2×1 shielded twisted pair.
Video signal transmission typically uses baseband frequency (around 8MHz), with coaxial cables being the most common medium. SYV75 series cables are standard, with SYV75-5 offering up to 300–500m non-relay transmission. For longer distances, SYV75-7 or SYV75-9 are used, which can extend transmission to over 1km.
Communication cables are used for remote control devices such as pan/tilt heads and motorized lenses. They often use 2-core shielded cables (RVVP) or Category 3 UTP with a cross-section of 0.3–0.5mm². RS-485 communication cables can extend up to 1200m, but in practice, RVV2-1.5 can extend beyond 2000m.
Control cables are multi-core cables used for controlling pan/tilts and motorized lenses. They are usually short (less than 1m), so shielding is not always necessary. Common models include RVV6-0.2 and RVV10-0.12. For longer distances, thicker cables like RVV10-0.5 or RVV10-0.75 are preferred.
Audio monitoring cables are typically 4-core shielded cables (RVVP) or Category 3 UTP with 0.5mm² per core. For point-to-point audio signals, unshielded 2-core cables like RVV2-0.5 can be used.
Signal cables between front-end detectors and alarm controllers often use RVV2*0.3 or RVV4*0.3. For alarm controllers and security centers, 2-core signal cables are common. The choice of shielded or unshielded cables depends on the brand’s specifications and the required signal quality.
Building intercom systems commonly use RVV, RVVP, and SYV cables. These cables support voice, data, and video transmission, with requirements varying based on the system's performance needs.
For direct button-type video intercom systems:
- Video terminals, two-way sound, and unlocking functions are connected via bus to the door unit, with individual call lines directly connected.
- Video coaxial cables: SYV75-5 or SYV75-3.
- Microphone/speaker/lock line: 4-core unshielded or shielded sheathed cable (AVVR4, RVV4, RVVP4).
- Power cord: 2-core sheathed cable (AVVR2, RVV2).
- Call line: 2-wire shielded cable (RVVP2).
For digital coded button video intercom systems:
- Main line: Video coaxial cable (SYV75-5, SYV75-3).
- Power cord: AVVR2, RVV2.
- Audio/data control line: RVVP4.
- Household signal line: RVVP6.
Key differences between weak current cables:
- **SYV vs. SYWV**: SYV is used for video transmission with polyethylene insulation, while SYWV is for RF transmission with physical foam insulation, commonly used in cable TV.
- **RVS vs. RVV 2-core**: RVS is a twisted pair of two-core RV wires without an outer jacket, used for broadcast connections. RVV 2-core has an outer jacket for power and control signals.
- **RVV vs. KVV / RVVP vs. KVVP**: RVV and RVVP use flexible wires made of multiple thin copper strands, while KVV and KVVP use hard wires composed of a single thick copper strand.
- **AVVR vs. RVVP**: AVVR refers to unshielded cables with wire diameters below 0.5mm, while RVVP refers to shielded cables with wire diameters ≥0.5mm.
- **RVS vs. RVV 2-core**: Same as above, emphasizing the difference in jacketing and application.