Circuit Protection Of Cables.

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Protection against overcurrent.

The term "overcurrent" means both overload current and short circuit current. The danger here is that the temperature rise in the conductors and their insulation will rise to levels at which the effectiveness of the insulation and its expected service life will be reduced.

The operating characteristics of a protective device must satisfy the following.

Ab must be equal to or less than An and equal to or less than Az.

Ab = current which the circuit is designed, eg, max demand.

An = nominal current of the protective device

Az = the continuous current carrying capacity of the conductor.

A2 = the current ensuring effective operation of the protective device and may be taken to for an MCB the conventional time for circuit breakers, 1.45 x An.

A = current in amps.

Example: A 20A MCB as a protective device on a 2.5sqmm circuit. Ab= 20A, An=20A, Az=20A and A2 = 29A.

An MCB will hold 1.2 x An at 40C, but should never be operated for long periods at this rating.

The actual current rating for cables depends on the following. ( single circuit configuration)

1. Method of installation.

Unenclosed in free air

Enclosed in conduit in free air.

Buried direct.

Enclosed in conduit, buried.

2. Ambient temp. This is normally 45C.

3. Type of insulation, PVC insulated and sheathed,75C, XPLE insulated and PVC sheathed 90C. These max operating temperatures must not be exceeded under fault conditions. Note: In Thailand cables are rated at 70C instead of 75C.

4. Voltage drop on the circuit.

The impedance of the circuit protected must be of a value that will trip an MCB in less than 0.4secs in the event of an earth fault. If this can not be achieved an RCD/RCBO must be utilised as the protective device.

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There is one point to mention on circuit protection and that is protection of consumers mains. In Thailand service protective devices are not generally used. This is generally an HRC fuse. It protects the service conductors, metering and consumers mains against short circuit, provides fault current limiting and provides a point for disconnection of the electrical installation.

They are not installed for overload protection,but will provide protection in certain conditions. Rated current of HRC fuse x 1.6. HRC fuses have a high fault current rating well in excess of the standard MCB.

The functions of the main MCB for the installation is to provide overcurrent protection and short circuit protection on the load side of the device only, it will not protect on the line side. Short circuit currents can be very high.

The minimum size of the main MCB in Thailand is generally 60/63A 10kA (220V) 6kA (380V) as an accepted standard, but often with consumers mains in older installations they may be found to be connected to 4.0sqmm, or 6.0 sqmm conductors.

Final sub circuits have a min of 5kA (220V) 3kA (380V).