As a supporting project of wiring engineering, there is no special specification for cable bridge at present and the manufacturer's specification program is lack of generality. Therefore, the suitable bridge and accessories such as M8 lag screw shield anchors should be selected reasonably in the design and selection process.
When it is allowed to use bridge system to form grounding trunk circuit, the grounding of bridge shall meet the following requirements.
1. The metal cable conduit leading in or out of the metal cable tray must be grounded (PE) or connected to neutral (PEN) and must meet the following specifications:
l The total length of the metal cable tray and its bracket shall be connected with the grounding (PE) or neutral (pen) trunk line at least 2 places, so that the whole bridge is an electrical path.
l The minimum allowable cross-sectional area of grounding wire at both ends of non-galvanized cable tray connection shall not be less than 4mm2.
l The two ends of the connecting plate between the galvanized cable trays may not be connected with the grounding wire, but the two ends of the connecting plate shall not be less than two connecting and fixing slotted hex head sleeve anchors with locking nuts or locking washers.
2. The coating shall be removed from the grounding hole and the special grounding washer with claw shall be used for the zinc plated drop-in anchors contacting with the coating.
3. The connection resistance between ladder ends shall not be greater than 0.000332. When equipotential bonding tester (conduction meter) or micro ammeter is applied, the test shall be conducted on both sides of the connection point. The connection resistance at both ends of the whole bridge should not be greater than 0.5Q, otherwise, grounding points should be added to meet the requirements.
4. The embedded slotted hex head toggle bolts shall be directly fixed on the M10 heavy-duty shield anchors with corresponding washers.
l When entering the building from outside, the outward slope of trough type cable bridge shall not be less than 1/100.
l The clear distance between cable bridge and electric equipment shall not be less than 0.5m.
l When two groups of cable bridges are laid in parallel at the same height, the clear distance shall not be less than 0.6m.
l The spacing, installation mode, model, specification and elevation of the bridge support points and slotted hex head wedge anchors can be listed on the plane, marked in sections and represented by different sections, single line drawings or detail drawings.
l The cable bridge should be more than 2.2m above the ground and the distance between the top of the cable bridge and the ceiling or other obstacles should not be less than 0.3m. The width of the bridge should not be less than 0.1m and the filling rate of the cross section in the bridge should not exceed 50%.
l When the cables in the cable bridge are vertically laid, the upper end of the cable shall be fixed on the bracket of the bridge by using carbon steel double expansion anchors and others. When laying horizontally, the cable should be fixed at the head, tail and turn of the cable by slotted hex head stud anchors.
l Cables shall be bound when laying in horizontal, vertical bridge and vertical trunking.