Highway Crossing on an 11 kV Spur: Clearance, Excessive Sag and a Cracked Pin Insulator
An 11 kV spur of about 3.2 km runs on 9 m PSC poles with Rabbit ACSR conductor in 60 m spans and crosses a state highway between two poles. After a thunderstorm the patrolling lineman reports: (i) the lowest conductor measures 5.4 m above the road surface; (ii) the mid-span sag on the crossing span, measured at 2 pm with the conductor at about 45 °C, is 1.35 m against a stringing-chart value of 0.90 m at that temperature; (iii) there is a brown flashover mark and a crack on the B-phase pin insulator on the crossing pole; (iv) that pole is a 30° section pole but has no stay. The spur is the only source for a water works, and the subdivision engineer wants it re-energised the same evening.
1. State whether the line can be re-energised the same evening as it stands, and justify your answer with reference to the clearance and the defective insulator.
Read the worked answer
No - two independent defects each bar re-energisation.
(a) Clearance: an 11 kV line crossing a public road must have a minimum clearance of about 6.1 m above the road surface (IER Rule 77 / CEA Measures for Safety and Electric Supply; confirm the current table in force). At 5.4 m the conductor is only 0.4–0.9 m above a loaded truck - a fatal-contact hazard and a reportable statutory violation, not an engineering preference.
(b) Insulator: a cracked porcelain pin insulator has lost its puncture strength. Under the pin's cantilever load the crack propagates, and in rain it will puncture and drop the conductor; the flashover track shows it has already been electrically stressed.
Sequence: keep the spur isolated with earths applied; replace the cracked insulator and, because the pole has been stressed, all three pin insulators on it; re-tension the crossing span to the chart sag (which itself restores clearance); re-measure clearance. If 6.1 m still cannot be achieved, insert an intermediate pole or fit a taller pole/extension before energising. Inform the control room and the water works of the duration and arrange a temporary supply if that consumer is critical - but do not trade clearance for speed.
2. The sag is 0.5 m more than the stringing-chart value. Explain what this indicates, its consequences, and the correct method of correcting it.
Read the worked answer
Sag and tension are inversely related: S = wL² / 8T for a level span. A sag 50 % above the chart value at the measured conductor temperature means the tension is only about two-thirds of design. Causes: under-tensioning during stringing (tension judged by eye or with an uncalibrated dynamometer), long-term creep of ACSR, a slipped binder or jumped disc, or sag read at a conductor temperature different from the one assumed.
Consequence: the conductor sits lower everywhere on the section and swings deeper in wind, so clearances to the road, buildings and any LT line below are all reduced - this is exactly why the crossing measures 5.4 m.
Correct method: read the required sag/tension for the actual span and the conductor's temperature from the stringing chart; work in the early morning when the conductor is cool and thermally stable; pull with a calibrated dynamometer/come-along and secure; re-measure sag and clearance and record both. Do not judge tension by eye or by feel, and remember that re-tensioning increases the longitudinal load on the pole and stay, so check them afterwards.
3. Why is the missing stay on a 30° section pole significant, and what else would you check on that pole before restoring supply?
Read the worked answer
On a section pole the two conductor pulls do not cancel. The resultant is 2T sin(θ/2) = 2T sin 15° ≈ 0.52T - about half the stringing tension acting permanently sideways, plus wind load. Without a stay the pole leans progressively, the cross-arm tilts, the clearance at the crossing falls further and the pole can fail in a storm.
Provide a stay in the direction of the resultant (roughly bisecting the external angle), set at about 45° to the ground, with proper anchor plate, stay rod, turnbuckle, egg insulator and 7/8 SWG GI stay wire; use double stays where tension is high.
Also check: foundation and plinth for cracks or erosion after the rain; the pole for cracks, leaning or exposed reinforcement; cross-arm condition; all insulators and binding wire; conductor strands for damage; the pole earth wire and its earth resistance; danger board and anti-climbing device. Finally establish the cause of the flashover - lightning (check for a line surge arrester and its earth) or a bird/animal bridge - because removing the symptom without the cause will repeat the failure.