What is a sheath gap?

A deliberate break in a cable's metal layers (armour, metallic sheath, moisture barrier), made with a gapping kit, so the metal is not one long continuous conductor.

Why: Long unbroken metal along the track picks up voltage from the traction supply and can carry traction or fault current into buildings, housings and equipment. Gapping keeps the metal sections short and stops that path at the building.

Where: Before the cable enters a building or lineside housing, and along the route wherever the continuous metal length would otherwise be too long for the traction system (AC rail-return, booster transformer or autotransformer areas). In some cases a joint can count as the gap (see 30162 cl 6.6).

Where the rules are

• NR/L3/TEL/30162 cl 6.6gapping the armour before a building or housing entry, and when a joint counts as the gap
• NR/L3/TEL/30162 cl 6.2what a gap physically is and the continuous-length trigger
• NR/L3/TEL/30162 cl 6.3AC rail-return areas
• NR/L3/TEL/30162 cl 6.4booster transformer and autotransformer areas
• NR/GN/TEL/30140 cl 4.3.8DI-SAC sheath gaps before housings
• NR/L2/TEL/30160 cl 6.4sheath gaps in the fibre design rules

What to order

• Armour Sheath Gap Kit · Graybar / Hybrid · GBK 1555, 086/001022
• Armour Sheath Gap Kit - 13-35mm Dia, Telecoms Cable · GBK 1529

PADS numbers with free sign-up on each part page. More in the parts finder

On the drawing

Shown at its position on the route on the SLD, and recorded in the survey and as-built records.

• NR/GN/TEL/30139 cl 7.8.5FTN items to show on the drawing, incl. sheath gaps
• NR/L2/TEL/31200 cl 8.4recording sheath gaps

Also called: armour gap, sheath break, armour gapping, sheath gapping, gapping kit

Guidance in our words. Read the current issue of each standard before you design to it.