Terminological Interpretation of TT, TN and IT Systems for Molded Case Circuit Breakers:

TT is a protective grounding system, that is, the protective system of direct grounding of metal shell of electrical equipment.

TN is a zero-connection protection system, that is, the metal shell of electrical equipment is directly connected with the zero-line protection system, which can be divided into TN-c and TN-s.

IT means that there is no grounding or high resistance grounding on the power side and grounding protection on the load side.

Application of TT, TN and IT systems for plastic case circuit breakers:

1. TT can be grounded directly by three-wire and three-phase ground wire. When a leakage fault occurs in a TT system, the fault voltage of the metal shell of the equipment is large, but the current is small, which is not conducive to the action of the protective switch and harmful to people and equipment. In domestic, it is limited to the occasion of electronic equipment with high local grounding requirements. At present, this system is not generally used in the construction site.

2. The commonly used system is TN system, i.e. three-phase four-wire system. The zero line of this system, i.e. the neutral line, is not allowed to be disconnected. This system can be further divided into TN-c, three-phase four-wire system, three-wire plus neutral line and TN-S three-wire plus ground line (both neutral line and ground line are shared) which are currently supplied by a single transformer or whose substations are relatively close to the construction site. The TN-S system is basically adopted in the construction foundation, which cooperates with the step-by-step leakage protection, and indeed plays a role in ensuring the safety of construction power consumption.

Note:

When using TN system, it is not allowed to disconnect the protective zero line.

3. There is also TN-C-S three-phase five-wire system, that is, three fire lines plus one neutral line and one ground line. TN-C-S system improves the safety of operators and equipment. TN-C-S system is usually adopted in construction site when the platform is far away from the site or there is no special transformer for construction.

Note:

1. TT system should be used in 10KV and 35KV high-voltage systems and some low-voltage power supply systems in mines and underground mines. It is not suitable for application in construction site.

2. TN-C is composed of three fire lines plus ground lines, and the neutral line and ground line are combined into one line. TN-S is composed of three fire lines plus neutral lines, which are separated from ground lines.