Live Sector, Open Edge: RF and Fall Protection on a Rooftop
A two-person crew is sent to a rooftop macro site to replace a corroded RRU jumper on sector 1. The roof is 24 m above ground with a 0.9 m parapet, and the antenna is on a 3 m pole at the roof edge. The site cannot be switched off: it carries the district's ambulance dispatch coverage. The escalation desk authorises work with sector 1 live at reduced power, 40 W per port, two ports, antenna gain 18 dBi, 2100 MHz. The crew's harness has a 2 m lanyard with no shock absorber, and the only certified anchor is 6 m from the work position. The DC plant is a 24-cell string floating at 54 V in a room on the same floor. A thunderstorm is forecast for late afternoon, and the crew leader wants to start immediately to finish before the rain.
1. Calculate the power density in the antenna main beam at 3 m and at 6 m, compare with the occupational reference level of 50 W/m², and state whether the job can be done in front of the antenna.
Read the worked answer
EIRP = 2 ports × 40 W × 10^(18/10) = 80 × 63.1 ≈ 5048 W.
Far-field power density S = EIRP/(4πd²):
- At 3 m: 5048/(4π × 9) ≈ 44.6 W/m²
- At 6 m: 5048/(4π × 36) ≈ 11.2 W/m²
Against the 50 W/m² occupational level, 3 m is nominally just inside and 6 m is comfortably inside. That is where the arithmetic stops being useful. The far-field expression requires the distance to be much greater than 2D²/λ. With a 1.4 m antenna at 2100 MHz (λ ≈ 0.143 m) the near-field boundary is around 27 m, so at 3 m and 6 m the crew is deep in the near field, where the simple formula can understate local hot spots by a large factor. Averaging over six minutes also assumes a person who can walk away, which is not true at height.
Answer: this job is not to be done in the main beam at any distance in front of the antenna. Approach from behind and below, keep the head and torso out of the beam axis, and if the jumper is unreachable from behind, ask for the sector to be muted. A short planned outage for a dispatch site is defensible; an RF burn at height is not.
2. Assess the fall-protection arrangement: anchor 6 m away, 2 m lanyard without a shock absorber, 0.9 m parapet, 24 m drop.
Read the worked answer
It fails on reach and on clearance, in that order of practical danger.
Reach: with the anchor 6 m from the work position and a 2 m lanyard, a worker cannot reach the antenna while attached. The realistic outcome on site is unclipped work at the roof edge, which is the actual fatal risk here, not the RF field.
Clearance: the work position is at the edge, so a fall over the 0.9 m parapet is a fall to ground, 24 m. A lanyard without a shock absorber transmits the arrest force straight into the body, well beyond the roughly 6 kN a person can survive, and it absorbs no energy. A 0.9 m parapet is not edge protection; guardrails are normally 1.0–1.1 m with infill that stops a person sliding under.
Controls needed before anyone goes up: guardrail or a horizontal lifeline along the edge giving continuous attachment; twin lanyards so one is always connected; energy-absorbing lanyard or self-retracting device with clearance calculated as lanyard length plus absorber deployment (about 1.0–1.5 m) plus 1 m safety; an anchor at or above shoulder height; a rescue plan with the means of rescue on site so a suspended casualty is recovered within about 10 minutes, because suspension trauma can kill a conscious, uninjured worker in that time.
If any of these cannot be provided, the job does not start.
3. Give the correct sequence of work from arrival to handover, and list the stop-work triggers.
Read the worked answer
Sequence:
- Pre-job: permit to work and job hazard analysis. Get the reduced-power authorisation in writing from the escalation desk, and check the lightning forecast.
- Isolate the RRU feed at the DC breaker, lock and tag it, and prove dead with a meter on the load side before touching the jumper.
- Treat the 54 V string with respect. It will not shock through dry skin, but a dropped spanner across a 24-cell string draws an arc that welds and burns. No jewellery, insulated tools, one-hand rule, and a mat over the terminals if work is near the room.
- Only now rig fall protection: anchor above shoulder height, twin lanyards, edge protection or lifeline in place, rescuer briefed and in position.
- Do the jumper change from behind and below the antenna, keeping out of the main beam.
- Close and weatherproof the connectors to torque, remove all tools, restore power, confirm the sector is radiating and check KPIs before signing off.
Stop-work triggers: thunder heard or lightning within 10 km; the anchor found uncertified or at the wrong height; the sector found still at full power or the muting request refused; the guardrail or lifeline not available; a person working alone at the edge; any change to the plan that nobody has assessed. The crew leader's wish to beat the rain is exactly the pressure that produces the incident - the forecast alone should have moved the start time.