344.1 Loop function & placement

Loops detect vehicles to authorize safe movement. Place within clear sight of the HVM bollard path and outside impact zones. Ensure a nearby crash rated bollard doesn’t shield the loop from metal mass.

An induction loop is a coil in the paving that shifts detector frequency when a vehicle’s metal mass is present. Use one or more loops to: (a) request access, (b) verify a clear zone before lowering/raising, and (c) hold an automatic bollard in a safe state if a vehicle sits above. Keep loops outside bollard impact envelopes and away from rebar-dense zones that can detune the system.

Entry/exit loops should align with lane flow and stopping points. A photo-eye or similar safety device can complement loops for non-metal objects and tailgating control; coordinate logic via the interlock matrix and I/O list.

344.2 Sizing & geometry

Use rectangles with aspect ~1:2; keep turns at 90°. Depth and spacing avoid rebar coupling (347). Correct geometry stabilizes HVM bollard lane logic.

Typical lane loops are 1.8–2.4 m long (traffic direction) by ~0.9–1.2 m wide. Maintain a consistent burial depth (40–60 mm in asphalt; 30–50 mm in pavers with sand/cement bedding) and a minimum 150–300 mm offset from reinforcement or metallic duct covers. Add 3–4 turns per loop window; more turns increase sensitivity but also raise noise susceptibility.

Separate adjacent loops by ≥1 m (edge-to-edge) to reduce cross-coupling. If close spacing is unavoidable, use different detector frequencies and twist feeder pairs at ≥6 twists/m to minimize crosstalk. In vehicle access control lanes, add a hold loop directly over the bollard throat only if paving and reinforcement layout allow clean margins.

344.3 Cable specification

Use twisted, screened PVC/XLPE rated for heat (337). Continuous runs reduce joints; specs must survive near a crash rated bollard base (334).

Select cable as a continuous “from detector to loop and back” circuit using oil/fuel-resistant sheath where vehicles may leak. For hot climates, specify XLPE or high-temp PVC insulation (≥90 °C rating) and UV-stable jackets for exposed risers. Use a shielded pair for feeders; bond the screen at the detector end only to avoid ground loops.

At interfaces with ducts and chambers, use raised cable glands and water-blocking compounds. Avoid splices inside paving; if unavoidable, use fully encapsulated resin kits rated for vehicular loading.

344.4 Feeder routing & joints

Route feeders in segregated ducts (246, 515). Resin-seal joints; avoid water traps (245). Dry routes prevent HVM bollard false triggers.

Run feeders in dedicated low-voltage/controls ducts separate from power to the PLC/Controller panel. Keep bends sweeping (no tight kinks), and avoid long parallel runs beside VFD/HPU cabling. Where ducts rise into pits or enclosures, fall the conduit away from the box or add drain/weep paths per the drainage strategy.

All joints should be above water line and fully potted. Label both loop tails and feeders with durable ferrules that match the integration documentation and asset register.

344.5 Detector tuning & filtering

Tune sensitivity/frequency to reject rebar and slow drift. Lock parameters in the I/O list (523). Stable tuning supports crash rated bollard interlocks (352).

Commission each detector by selecting a frequency channel with minimal cross-talk, then set sensitivity high enough to catch motorcycles but low enough to ignore carts and rebar. Use presence-timeouts and undetect hold-times that suit throughput targets defined in your KPIs. For multi-loop logic, debounce the Request→Authorize→Execute chain in the control logic.

Record the final gain, frequency, filter modes, and presence timing into the I/O list and panel door schedule. Where hardware supports it, enable self-retune and drift alarms (see 541).

344.6 Environmental influences

Heat, sand, and moisture shift baselines (337). Add auto-retune where supported. HVM bollard reliability improves with monitored drift alarms (541).

In GCC climates, thermal expansion of paving and moisture ingress can change loop inductance, causing slow baseline drift. Shield feeder entries, keep chambers dry, and specify enclosures with suitable IP rating. For coastal sites, protect splices against saline attack and plan quarterly re-verification as part of the preventive maintenance plan.

Where loops traverse stone modules or decorative metals, check for eddy-current effects and re-tune after surface reinstatement (629). If approvals are in scope, note that SIRA reviewers may ask to witness loop behavior; include a concise note and link to SIRA Bollards (UAE).

344.7 Testing & proving methods

Prove with calibrated test wheels and metal blocks (633). Record values on SAT forms (638). Tests show a crash rated bollard lane reacts predictably.

Before traffic, verify coil resistance and insulation with a megger at safe voltages (per detector manual). Then prove live detection using a calibrated metal test piece (e.g., “motorcycle surrogate”) and a vehicle roll-through at walking speed to confirm presence, hold, and undetect timings. Capture screenshots/values in the Loop & Sensor Proving sheet and cross-reference the SAT / Witness Procedure.

Finally, run the interlock verification (634) to ensure loop states actually prevent unsafe movement during bollard raise/lower, including EFO overrides.

344.8 Fault finding checklist

Check supply, loop resistance, detuning from rebar, and ground faults. A short, ordered list speeds HVM bollard recovery.

• Power & wiring: confirm detector supply, terminal tightness, and screen bonding (detector end only).
• Loop health: measure resistance/inductance vs. baseline; inspect for cut or water-filled joints.
• Noise sources: separate feeders from HPU/VFD power; change frequency channel if cross-talking.
• Rebar/metal influence: re-tune after any paving/core drill near the loop.
• Controls mapping: verify inputs are latched/mapped correctly in the PLC and shown on the HMI.

344.9 Documentation requirements

Capture loop plans, settings, and serials (539, 716). Docs defend crash rated bollard safety behavior during review (444).

Include: (a) loop layout drawings with dimensions, depth, turns, feeder route, (b) cable schedule and test results, (c) detector brand/model, firmware, and final tuning parameters, (d) photos before pour and after cut/seal, and (e) commissioning sheets (633) plus witness records (638). File these under the Evidence Capture Standards and index them in the Handover Pack Index.

344 Induction Loops — FAQ