1. Analysis of the causes of the problems and preventive measures
1.1 Lightning protection grounding.
1.1.1 Phenomena: Defects such as slag inclusion, weld beads, incomplete welds, undercut, and incomplete welds are found at the joints of down conductors, equipotential rings, and lightning protection strips; weld slag is not removed, and anti-rust paint is not applied to the welded joints of the lightning protection strips; threaded steel is used instead of round steel as lap reinforcement.
1.1.2 Cause Analysis: The operators were not skilled in welding techniques; the on-site construction manager did not fully comply with the GB50169-92 Specification for Construction and Acceptance of Grounding Devices in Electrical Installations.
1.1.3 Preventive measures:
(1) Strengthen the skills training of welders, especially for high-difficulty welding such as vertical welding and overhead welding.
(2) The connection of the lightning protection down conductor is lap welding, and the lap length is 6 times the diameter of the round steel. Therefore, it is not allowed to use threaded steel instead of round steel as lap reinforcement. In addition, if the main reinforcement of the down conductor is butt welded, a lap round steel should be added at the butt weld as required.
1.2 Pre-embedded outdoor inlet pipe.
1.2.1 Phenomena: Thin-walled steel pipes are used instead of thick-walled steel pipes; the pre-embedded depth is insufficient and the position deviation is large; the bends are bent by electric welding, and the pipes on the wall are welded to the horizontal inlet pipe network at a 90° angle; the waterproofing treatment of the inlet pipe and the basement exterior wall is not good.
1.2.2 Cause Analysis: Material procurement staff are not familiar with national standards when making purchases; some construction units deliberately confuse them to reduce costs; construction managers are not strict or dare not object to the contractor's deliberate violations; and there are loopholes in the management of materials entering the site by the supervision personnel.
1.2.3 Preventive measures:
(1) Thick-walled steel pipes must be used for the pre-buried pipes entering the house.
(2) Strengthen coordination and cooperation with civil engineering and other disciplines, and clarify the outdoor ground elevation.
(3) The bends of the pre-embedded steel pipes on the wall must be bent with a pipe bending machine or a special 9-fold bend must be purchased. Welding and hot welding are not allowed for bending. After bending, the steel pipe should not have cracks or significant dents. The degree of bending should not be greater than 10% of the outer diameter of the pipe, and the bending radius should not be less than the minimum allowable bending radius of the cable to be inserted.
(4) Ensure proper waterproofing.
1.3 Installation of electrical conduits (steel pipes, PVC pipes).
(1) Phenomena: Multiple layers of electrical conduits are overlapped; the depth of the conduits buried in the wall is too shallow, or even buried in the putty layer outside the wall. The conduits have dead bends, folds, and dents; when the conduits enter the distribution box, the conduit opening is not smooth inside the box and is too long; the conduit opening is uneven and of different lengths; no protective rubber ring is used for the conduit opening; when pre-burying PVC electrical conduits, the conduit opening is not blocked with a plug, but flattened and bent with pliers.
(2) Cause analysis: Insufficient coordination between architectural design and electrical engineering resulted in multiple conduits passing through the same narrow plane.
(3) Preventive measures: When there are more than 6 units per floor in the tower, the civil engineering should preferably use the decoration method of the ceiling of the public corridor. Most of the electrical entry lines can be directly connected to the residents through the cable trays laid above the ceiling. Alternatively, the floor slab of the public corridor can be thickened to conceal the numerous electrical conduits. The electrical conduits should not be placed side by side or close together. The conduits should be buried in the brick wall and should be no less than 15mm from the surface. The conduits should be laid horizontally and vertically. a. The bending radius of the conduit (concealed) should not be less than 10 times the outer diameter of the conduit. The conduit should be bent with a pipe bending machine or spring to make the bend smooth. b. The conduit should be flat when entering the distribution box, with an exposed length of 3mm to 5mm. The conduit opening should be covered with a protective sleeve and the box shell should be locked. For electrical conduits entering floor-mounted distribution boxes, the conduit opening should be 50mm~80mm higher than the foundation surface of the distribution box. When pre-burying PVC electrical conduits, it is forbidden to use pliers to flatten or bend the conduit opening. Instead, use PVC plugs that match the conduit diameter to seal the conduit opening and secure it with tape.
1.4 Wiring, connection quality, and color coding of wires.
1.4.1 Phenomena: Multi-strand wires are not connected with copper connectors and are directly made into "sheep's eye" shapes; when connecting to the terminals of switches, sockets, and distribution boxes, several wires are connected to one terminal; wire ends are exposed, wires are not neatly arranged, and there is no binding or wrapping; the color codes of the three phases, neutral (N) wire, and grounding protection wire (PE) are inconsistent or confused.
1.4.2 Cause Analysis: Construction personnel were not proficient in wire wiring techniques and methods. The material procurement officer did not prepare sufficient quantities and colors of all required wires, or construction management personnel mixed different types of wires to save on materials.
1.4.3 Preventive measures:
(1) For the connection of multiple wires, galvanized copper connectors should be used for crimping. Only one wire should be connected to each terminal block and terminal block. If two wires need to be connected, a flat washer should be added in between.
(2) The wires should be arranged horizontally and vertically. When stripping the wire ends, the length of each wire end should be consistent. After the wires are inserted into the terminals, there should be no exposed conductors. The connection between the copper connector and the wire should be wrapped with insulating tape of the same color as the wire.
(3) Procurement personnel should provide sufficient wires of various colors according to site requirements. Construction personnel should distinguish the function and color code of phase wire, neutral wire (N wire), and protective earth wire (PE wire).
1.5 Installation and wiring of the distribution box.
1.5.1 Phenomena: There are gaps between the enclosure and the wall; the enclosure is not straight; debris inside the enclosure has not been cleaned properly; the openings in the enclosure do not meet the requirements, especially those made by electric welding or gas welding, which seriously damage the paint protection layer and the appearance of the enclosure; the grounding of the floor-mounted power box is not obvious, and the cross-section of the repeated grounding wire is insufficient; the wire ends inside the enclosure are exposed, the wiring is not neat, and the wires do not have enough slack.
1.5.2 Cause Analysis: Insufficient coordination with civil engineering during enclosure installation; incomplete filling of gaps by civil engineering; and failure to use a level to calibrate the enclosure during installation.
1.5.3 Preventive measures:
(1) Carefully clean the mortar and debris inside the box.
(2) Strictly specify the dimensions when ordering and produce according to the dimensions to ensure that the "knockout hole" opening of the box matches the inlet pipe. If they do not match, the hole must be mechanically drilled or the box must be sent back to the manufacturer for reprocessing.
(3) The grounding point and wires of the power box must be clearly exposed and must not be welded or wired under the box. The wire ends inside the box must be uniform and not exposed. The wiring must be neat and beautiful, and tied and fixed. The wires must have a certain amount of slack, generally 10cm to 15cm slack inside the box.
1.6 Installation and wiring of switch and socket boxes and panels.
1.6.1 Phenomena: The junction boxes are buried too deep and the elevation is inconsistent; there are gaps between the panel and the wall; the panel is contaminated with glue and paint and is not straight; there is mortar debris in the junction boxes; the phase wire, neutral wire and PE protective wire of the switches and sockets are connected in series; the wire ends of the switches and sockets are exposed; the fixing bolts are loose; and there is insufficient wire slack in the box.
1.6.2 Cause Analysis: The pre-embedded junction box was not securely fixed, the template bulged, and the coordinates were inaccurate during installation.
1.6.3 Preventive measures:
(1) Work closely with the civil engineering team to ensure accurate and secure fixing of the junction boxes. If the pre-embedded junction box is too deep, an additional junction box should be installed. When installing the panel, ensure it is horizontal and vertical, and use a level to adjust the level to ensure uniform installation height.
(2) Strengthen management and supervision to ensure that the phase wire, neutral wire and PE protection wire in switches and sockets are not connected in series, and clean the mortar in the box first.
(3) When stripping the wire, fix the size to ensure that the wire ends are neat and uniform and that the wire ends are not exposed after installation. In order to firmly press the wires, single-core wires should be bent into double strands when inserted into the wire hole and tightened with screws; wires in switch and socket boxes should have a certain amount of slack, generally at least 100mm~150mm.
2. Conclusion
Building electrical engineering depends on buildings for its existence and use, and is closely related to people's daily production and life. Its quality directly affects the safety and usability of building projects. Data from relevant departments shows that electrical fires rank first among all types of fires in my country every year, and accidents involving electric shock and damage to electrical equipment also occur frequently.
