What is the difference between an isolated power system and non-isolated power system?

1. Introduction

There are three basic types of low voltage power distribution grounding and zeroing systems: IT, TT, and TN. TN is divided into three derived types: TN-C, TN-S, and TN-C-S.

The first letter indicates the state of the power supply neutral point with respect to ground:

T—indicates that the neutral line of the power supply is directly grounded; I —indicates that the neutral line of the power supply is not grounded or is grounded with high impedance.

The second letter indicates the state of the load side relative to ground:

T — The externally leaking conductive parts of the electrical device are directly grounded, and the grounding point is independent of the grounding point at the power supply end;  

N — The exposed conductive parts of the electrical device have a direct electrical connection to the grounding point of the power supply terminal.

The third letter C- indicates that the neutral line (N line) and the protective zero line (PE line) are shared; the fourth letter S- indicates that the neutral line (N line) and the PE line are separated.

1.1 IT System

Definition: The power supply is not grounded, and the equipment housing is grounded.

Protection: The insulation resistance of the monitoring system to ground.

1.2 TT System

Definition: Power supply grounding, equipment housing grounding.

Protection: Monitoring system residual current to ground.

1.3 TN-C System

Definition: Power supply and equipment housing grounding. Two grounds connected and neutral ground separated.

Protection: Monitoring system residual current to ground.

1.4 TN-S System

Definition: Power supply grounding, equipment housing grounding. Two grounds connected, zero ground combined.

Protection: Monitoring system residual current to ground.

2. Comparison between IT system and TN system

2.1 TN system: Poor power supply safety and continuity

In a grounding system, when a live wire grounding fault occurs, the neutral live wire is short-circuited to the ground, and the circuit breaker trips, affecting the continuity of power supply.

Any electrical equipment powered by the TN system will have a certain amount of leakage, which will pose a threat to medical staff and patients.

If the leakage current of the equipment is too large and exceeds the operating current of the leakage current protection circuit breaker, the circuit breaker will trip, which will also affect the continuity of power supply.

2.2 IT system: Good power supply safety and continuity

The IT system is not grounded. When any live wire fails to ground (the first ground fault), the insulation monitor reports a ground fault, but the circuit breaker will not trip, and the system can continue to operate with the fault.

Because the system is not grounded, the leakage current forms a loop through the system distributed capacitance. Under high capacitance, the leakage current will be relatively small, and the normal system leakage current is below mA.

For medical IT systems, the IEC standard requires that the input and output circuit breakers of the IT system are not allowed to have overload protection functions, and the overload will not be cut off.

3. Solution

Medical IT System Insulation Monitoring Products