Tower cranes are a critical building tool.
For many urban construction jobs to begin, tower cranes need to be assembled onsite well in advance. Much like the buildings they are used to construct, an assessment need to be performed before the crane is put into operation. Once assembled, work can only be started after the crane is approved by the Authority Having Jurisdiction (AHJ).
Even if the crane was recently evaluated at a different job site, the AHJ (OSHA and city inspectors) will require the crane to be evaluated. Since the crane has been disassembled and moved to a new job site, any earlier evaluations and certifications are nullified.
Tower crane evaluation presents a unique challenge. Once the crane is relocated, a re-evaluation starts at the new job site. In addition to the challenges in field evaluation of new or used equipment at facilities, tower cranes require particular attention when assessing their cables, especially when it comes to their sunlight resistance rating and the specific type of overcurrent protection that exists for the motors, cables and variable frequency drives.
While certain types of motors on the crane are considered "short-time" (the hoist motor, for example) and don't require overload protection, other motors, such as the ventilation motors, do.
Tower crane evaluations can present other unique challenges in the field that are often not encountered with other new or used equipment, such as:
Missing or outdated electrical drawings
Electrical drawings for tower cranes are often not readily available, are not updated with modifications that may have occurred with the equipment or are not legible given the age of the equipment. Accurate electrical drawings are required for all equipment. If they are not available or are not accurate, an updated electrical drawing must be created before the equipment can be labeled.
Inaccurate or insufficient wiring.
Wiring within tower cranes is often modified from the original design. This can include the wiring in panels or within the interconnecting cables between the components of the crane. The wiring must be verified to be suitably sized and have the appropriate insulation type for the application.
Inaccurate or insufficient wiring could become a serious safety hazard. Wiring which is not properly rated, routed and approved for the application can be damaged by conditions such as overcurrent, physical damage and exposure to the environment.
Manufacturer interface
Often the original tower crane manufacturer is overseas and technical information, including machine drawings or a bill of material are not readily available or easily obtained.
Damaged components
Components of the equipment can be damaged by a variety of causes. Common damage seen in tower crane components include:
- Wiring exposed at strain relief bushings that is no longer secured at the jacket.
- Internal wiring not adequately trained and bundled in approved Panduit.
- Enclosure door interlocks are frequently damaged and not operating correctly.
- Conduits, fittings and various electrical components such as motor contactors, motor overload relays and overcurrent protective devices are often found to be damaged from heat or not adequately secured to the enclosure wall.
Suitability of installation
When a tower crane is relocated to a new job site, the location selected for the crane to be installed must also be evaluated. Dusty environments or atmospheres where flammable gases or vapors are concentrated enough to produce an explosive or ignitable mixture, must be taken into consideration to determine the suitability of the installation.
What electrical standards apply?
The appropriate product electrical safety standard is the primary standard used for evaluating electrical equipment. Where no single product standard exists, the applicable portions of several standards are combined into the reference product standard.
What is inspected and tested?
The purpose of an evaluation is to ensure that the crane is assembled to the manufacturer's specifications and that all electrical system components are sufficient for the strain put on them during operation. Completing an evaluation of the tower crane electrical system prior to use after a move will protect your equipment investment as well as your personnel.
The parts or components of a tower crane that are tested and inspected during an evaluation are as follows:
- Proper nameplate information
- Proper component identification
- Damaged components
- Inspection of components for evidence of (NRTL) listing or recognition
- Guarding of live parts within an enclosure
- Grounding
- Overcurrent protection
- Wiring methods and ratings
- Safety interlocks and emergency off or emergency stop controls
- Environmental suitability
- Testing may include but not be limited to: heat rise testing, dielectric withstand, leakage current, interlock and E-Stop functional testing
Safety designations
Typical issues are easy to miss. Absent or incorrectly labeled control panel nameplates, crane disconnection markings, incorrectly colored conductors and missing hazardous voltage and arc flash warnings signs are among the usual infractions that need to be addressed. These may seem like small infractions, but they can make the critical difference between safe operation of the equipment and a fatal one. To meet the applicable safety standards and ensure the safety of employees and the public, it is essential that the cranes are correctly assembled and installed.
Once the crane has been evaluated and verified to be in compliance with applicable electrical standards, the accredited organization performing the field evaluation will affix their label on the equipment, and a final report is issued to the AHJ and the client detailing the assessment performed. Final approval is up to the AHJ.
Conclusion
Keeping a tower crane in service and at peak operating condition should be a top priority for the safety, maintenance and operations personnel at any job site. Frequent and periodic inspections help keep crane equipment working at maximum efficiency, keep the user and other personnel safe, reduce costly downtime and extend the life of the equipment.
This story also appears on Crane Equipment Guide.
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