Overhead Crane Safety System Design: Circuit & Anti-Collision
Key Points The crane safety protection system comprises three main parts: ① Electrical safety circuit (redundant dual channel, PL d~PL e, Emergency Stop cutoff ≤200ms); ② Mechanical safety devices (Overload Limiter combined error ≤±5%, Hoisting Height Limit Switch trigger point ≤20mm, Travel Limit Switch buffer ≥1.5m); ③ Protection and interlocking (IP23/IP44 indoor, IP54/IP55 outdoor, Grounding Resistance ≤4Ω, anti-collision detection 0.5~20m). The system is designed to ISO 4301 and GB/T 28264 Safety Monitoring and Management System, and any single trigger from the three interlocked systems brings the entire crane to a stop.
The crane safety protection system consists of three main parts: the electrical safety circuit, mechanical safety devices, and protective interlocking. Every crane shipped from Kelude is equipped with a safety protection system designed to ISO 4301 and GB/T 28264 Safety Monitoring and Management System. The design must also comply with GB/T 16855.1 (Safety of machinery — Safety-related parts of control systems). This article provides an engineering-focused overview of the safety system design process, key component selection parameters, and common troubleshooting approaches.
For fundamentals and frequently asked questions on safety protection, see the Complete Guide to Crane Safety Protection. For intelligent safety protection system configuration, refer to the Overhead Crane Intelligent Safety Protection System Configuration Guide. For an interpretation of the Safety Monitoring and Management System standard, see GB/T 28264 Standard Explained for Overhead Crane Safety Monitoring.
The three main components of a crane safety protection system
Designing the Electrical Safety Circuit
The safety circuit uses a redundant dual-channel architecture — the Hoisting Height Limiter, Overload Limiter, and Emergency Stop each have two independent signal paths, and the two channels are combined with an AND logic output (the output is valid only when both channels are healthy). If either channel fails, the system performs an automatic shutdown and displays a fault code. The safety performance level must reach PL d~PL e per GB/T 16855.1 (corresponding to a probability of dangerous failure per hour of ≤10⁻⁷). Emergency Stop buttons are red mushroom-head, self-locking type; the time from pressing the button to the Contactor main contacts opening is ≤200ms. Recommended Safety Relays include the Pilz PNOZ series or SICK Flexi Classic, with contact status indicated via LED and quarterly checks for contact welding.
Selecting Mechanical Safety Devices
The Overload Limiter is selected to GB/T 12602-2020, with a combined electronic error of ≤±5% and the trip point set between 100% and 110% of the rated load. Hoisting Height Limit Switches come in two types: counterweight type (simple and reliable but lower accuracy) and screw type (higher accuracy and longer service life). The limit switch triggers when the empty hook rises to a point where at least 2 wraps of rope remain on the Drum, with an upper deviation of ≤20mm at the trigger point. For the Crane Bridge and Trolley Travel Limit Switches, a buffer distance of ≥1.5m is reserved, or the braking distance × 1.25 is used for the calculation. Wind Rail Clamps are available in manual type for small-to-medium spans and electric type for large spans in high-wind regions, with clamping force calculated at 1.5 times the design wind load.
Protection Rating and Earthing Protection
Electrical Protection ratings follow GB/T 4208-2017: indoor, normal temperature environments use IP23 (protection against ≥12.5mm solid objects) or IP44 (protection against ≥1mm objects plus splashing water); outdoor or exposed installations use IP54 (Dustproof plus splashing water) or IP55 (Dustproof plus Water Jet Proof); dusty environments such as woodworking, ceramics, or similar facilities use IP6X (totally Dustproof) plus IP65 (Water Jet Proof). Earthing Protection uses a TN-S three-phase five-wire system, with all metal enclosures connected to the PE conductor and a Grounding Resistance of ≤4Ω (measured with a ZC-8 earth tester). The earthing busbar uses 40×4 galvanized flat steel or ≥16mm² copper-core wire, with a Welding length of ≥100mm.
Anti-Collision and Safety Monitoring System
When multiple cranes operate on the same runway, an Anti-Collision System is required. Sensor options include: LiDAR (high accuracy, 10~20m range), infrared (medium accuracy, 5~15m), and ultrasonic (lower accuracy but unaffected by dust, 0.5~5m). The Safety Monitoring System, per GB/T 28264 Safety Monitoring and Management System, monitors parameters such as Lifting Capacity, Lifting Height, travel distance, and wind speed (for outdoor cranes) in real time, with data storage of ≥72 hours. The monitoring system is interlocked with the safety circuit: when any safety device is triggered, the monitoring system logs the event and sounds a voice alarm.
| Safety Device | Type | Critical Parameter | Applicable Standard |
|---|---|---|---|
| Safety Relay | redundancydual channel | PL d~PL e, Cut-off≤200ms | GB/T 16855.1 |
| Overload Limiter | Electronic Type/Machinery Electronic Type | Combined Error≤±5%, Alarm110% | GB/T 12602-2020 |
| Hoisting Height Limiter | Gravity Type/Screw Type | At Trip Point Deviation≤20mm | ISO 4301 Crane Design Standard-2008 |
| Travel Limit Switch | Lever Type/Cam Type | Buffer≥1.5morbraking distance×1.25 | ISO 4301 Crane Design Standard-2008 |
| Wind Rail Clamp | Manual/Electric | Clamping Force≥1.5Wind Load Factor | ISO 4301 Crane Design Standard-2008 |
| Anti-Collision Device | Laser/Infrared/Ultrasonic | Detection Distance0.5~20m | GB/T 28264 Safety Monitoring and Management System-2017 |
Related Standards
• GB/T 24818.4-2009 — Cranes — Access and Safety Devices — Part 4: Jib Cranes
• GB/T 6974.6-2008 — Cranes — Vocabulary — Part 6: Railway Cranes
• GB/T 6974.7-2008 — Cranes — Vocabulary — Part 7: Floating Cranes
Frequently Asked Questions
Q: Can a safety relay be replaced with a standard relay?
A: Absolutely not. Safety relays feature forced-guided contacts that prevent normally open and normally closed contacts from being closed simultaneously, and they include built-in self-diagnostic functions. A standard relay can fail if its contacts weld or stick, and using one in a safety circuit can lead to a serious accident. Safety circuits must use certified safety relays.
Q: What IP rating should an outdoor gantry crane have?
A: The electrical cabinet must be rated at least IP55 (dustproof and water jet proof). For coastal or high-humidity environments, IP65 (dustproof and powerful water jet proof) is recommended. Motors should be rated IP54 or higher, and junction box connections should be sealed with rubber gaskets. Fit the control cabinet with a heated dehumidifier (50–100 W, auto-start at ≥70% humidity) to prevent condensation-induced short circuits.
Q: Is a 5% tolerance on the overload limiter acceptable in practice?
A: The ±5% overall error required by GB/T 12602-2020 covers the full chain including the sensor, transmitter, and display. In practice, Kelude calibrates its overload limiters to within ±3% at the factory, and the trip points (alarm/shutdown) are set at 105%–108% of rated load. This ensures safety without causing frequent nuisance shutdowns from minor overloads that would interrupt production.
Q: How much safety distance is actually required?
A: Safety distance = braking distance × 1.25 + buffer stroke. Kelude calculates the safety distance based on actual parameters during the factory acceptance test and marks it on the crane nameplate. For a 32t/30m gantry crane as an example: long-travel speed v = 25 m/min ≈ 0.417 m/s, braking deceleration a = 0.2 m/s², system response time t = 0.1 s, braking distance = v²/(2a) + t × v = 0.434 + 0.042 = 0.476 m. Safety distance = 0.476 × 1.25 + 0.3 (buffer stroke) = 0.895 m, rounded up to ≥1 m in engineering practice. For gantry cranes with a larger dead weight, ≥1.5 m is recommended.
