In the laying of light and power cables every joint, as made, should be tested for insulation with a Thomson galvanometer, as the insulation must necessarily be very high, and if one joint or section of cable is any weaker than another it may be very important in the future to know it.  All tests must be made after the joint has cooled, for while hot its insulation resistance will be very low.

Tests for copper resistance should also be made to determine if the splices are electrically perfect; an imperfect splice may cause considerable trouble.  In telegraph and telephone cables the conductors should be of very soft copper, for in stripping the conductor of insulation it is very easy to nick the wire, and if of hard drawn copper open wires will be the result.

All work should be frequently tested for continuity with telephones, magnetos, or small portable galvanometers.  It is only necessary to ground the conductors at one end and try each wire at the other end.  For this sort of work a telephone receiver used with one cell of some dry battery is most convenient, and has the additional advantage of affording a means of communication while testing, and is by far the best thing for identifying and tagging conductors.

These cables should be frequently tested during the progress of the work for grounds and crosses with a Thomson instrument, and when the cable is complete, a careful series of tests of the capacity, insulation resistance, and copper resistance of each wire should be made and the exact condition of the cable determined before it is put in service, and thereafter an intelligent oversight of the condition of the circuits can thus be more readily maintained.

Where a company has extensive underground service, a regular cable gang should be in its employ, for quick and safe handling of cables demands the employment of men accustomed to the work.  If the cable has been properly laid and tests show it to be in good condition before current is turned on, almost the only trouble to be anticipated will be due to mechanical injury.  Disruptive discharge, puncturing the lead, may occur; but the small chance of its occurring can be greatly lessened by the use of some kind of “cable protector,” which will provide for the spark an artificial path of less resistance than the dielectric of the condenser, which the cable in fact becomes.

If a fault suddenly develops on a circuit, the chances are it will be found in a manhole, and an inspection of the cable in the manhole will generally reveal the trouble without resorting to locating with a Wheatstone bridge.  The cable is often cut through at the edge of the duct, or damaged by something falling on it, or by some one “walking all over it.”  To guard against these, the ducts should always be fitted with protectors both above and below the cable.  The cables should never be left across the manholes, for they then answer the purpose of a ladder, but should be bent, around the walls of the hole and securely fastened with lead straps, that they may not be moved and the lead gradually worn through.