Understanding cable system configurations …
The three cables that make up a 3 phase circuit can be placed in different formations. Typical configurations include the trefoil or triangular and the flat formation. The choice will depend on several factors like screen bonding method, conductor area and available space for installation.
Typical cable formations
The electric power losses in a cabling circuit are dependent on the currents flowing in the metallic sheaths of the cable. Reducing or eliminating the sheath currents through bonding methods will allow the load current carrying capacity (ampacity) of the cable circuit to be increased. Typical bonding methods are Both-end bonding, Single-point bonding and Cross-bonding.
A system is both end bonded if the arrangements are such that the cable sheaths provide path for circulating currents at normal conditions. This will cause losses in the screen, which reduce the cable current carrying capacity. These losses will be smaller for cables in trefoil formation than in flat formation with separation.
Both end bonded
In single point bonding the screens are connected and earthed at one end of the route. At all other points, the screen being insulated from earth will have a standing voltage which will be proportional to the circuit length, conductor current and cable spacing, and be at a maximum value at the furthest point from the earth bond. Since there is no closed circuit, screen circulating current is eliminated. Single point bonding is normally used for limited route lengths to keep the standing voltage to the minimum and render the cable installation safe against “touch-voltage”.
Single point bonded
Cross bonding consists of sectionalizing the cable screen into sections called minor sections and cross connecting them so as to neutralize the total induced voltage in three consecutive sections. Three minor sections together make a major section. In cross bonding system, the route is split up into groups of three drum lengths with the screens bounded and earthed together at both ends of a major section, but interrupted and connected in series at all other points. The purpose is to allow a standing voltage between screen and earth in each major section but eliminate circulating currents. With this arrangement, the current carrying capacity can be considerably enhanced particularly for large conductor sizes and further, application is possible for longer route lengths.
After installation the sheath test is the first test which should be performed. This test will indicate whether there has been any mechanical damage to the outer cable sheath. If there has been damage to the sheath the degradation of the neutral and the cable insulation will be imminent which will result in premature cable failure.
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Editor’s Note: This post was originally published July, 2011 and has been updated freshness, accuracy and comprehensiveness.