Intrinsic safety – I.S is a protection technique for the safe operation of electrical and electronic equipment/system in Hazardous Areas where any ignition source needs to be eliminated.

Electrical cables are essentially sealed from damage, external electrical or magnetic fields (EMI), and isolated from non-intrinsically safe circuits. (Avoid the increase of amperes mA in I.S system –> A –> Hot spot –> Ignition -> Explosion )

I.S cables are usually colored blue so as to identify the potential risk of the electric circuit and the need for special consideration.

• The I.S cable to be separated from Non-I.S circuits. 

Hazardous areas is the key point to give serious concern on this cable installation.

This term most noticed on tanker/product or Chemical/LPG/ ect where has Hazardous areas.

Above is an typical example for I.S cable installation common issue on chemical tanker, the Block located on Fwd – Accommodation part placed in Hazardous area (Also A-60 class to be complied).

As require in ABS-rule Part 4 captures:

Cables in Hazardous Areas (2006)
Cables in hazardous areas are to be armored or mineral-insulated metal-sheathed, except for cables of  intrinsically safe circuits subject to the requirements of 4-8-4/21.15. Where cables pass through boundaries of such locations, they are to be run through gastight fittings. No splices are allowed in hazardous areas, except in intrinsically safe circuits.

“21.15.2 Additional Protection
Cables installed in locations such as within cargo holds, in way of cargo hatch openings, open decks subjected to seas, etc., even of the armored type, are to be protected by substantial metal shields, structural shapes, pipe or other equivalent means, which are to be of sufficient strength to provide effective protection to the cables.”

“For cable penetrations through watertight, gastight, and fire-rated bulkheads and decks, evidence of penetration design approval is to be submitted. For watertight and gastight cable penetrations, certificates issued by a competent independent testing laboratory would be acceptable. For fire-rated cable penetrations, certificates issued by an administration signatory to SOLAS 1974 as amended would be acceptable.” 

Installation of Cables and Wiring (2005)
Installations with intrinsically safe circuits are to be erected in such a way that their intrinsic safety is not adversely affected by external electric or magnetic fields — MEAN OF REGARDLESS TO THE ZONE under normal operating condition and any fault conditions, such as a single-phase short circuit or earth fault in non-intrinsically safe circuits, etc.

Separation and Mechanical protection for IS system IN HAZARDOUS AREA – DOESN’T MEAN REGARDS TO THE ZONE

The installation of the cables is to be arranged as follows:
i) Cables in both hazardous and non-hazardous areas are to meet one of the following
requirements:
• Intrinsically safe circuit cables are to be installed a minimum of 50 mm (2 in.) from all non-intrinsically safe circuit cables, or
• Intrinsically safe circuit cables are to be so placed as to protect against the risk of mechanical damage by use of mechanical barrier, or
• Intrinsically safe or non-intrinsically safe circuit cables are to be armored, metal sheathed or screened.
ii) Conductors of intrinsically safe circuits and non-intrinsically safe circuits are not to be carried in the same cable.
iii) Cables of intrinsically safe circuits and non-intrinsically safe circuits are not to be in the same bundle, duct or conduit pipe.
iv) Each unused core in a multi-core cable is to be adequately insulated from earth and from each other at both ends by the use of suitable terminations.

For full comprehension, pls see the Ship Hazardous Zone and Structure integrity  (INSULATION) articles.