A colleague recently asked what the power distribution system designations “IT” and “TN” mean. The “power distribution system” is all parts of an electric system between the “bulk power source” and the consumer’s service-entrance equipment.
For the purposes of this discussion, the “bulk power source” is the secondary of a power distribution transformer, where the output voltage is the utilization voltage, commonly taken as 100, 120, 127, 220, 230, or 240 volts.
There are three basic power distribution systems, TN, TT, and IT. Within the TN system, there are three variations, designated TN-S, TNC, and TN-C-S. We will examine what these designations mean, and how they affect safety of the system and of products connected to the system.
Also, we will define “polarization” as it applies to the power distribution system and to the safety of products.
And, we will discuss power distribution system grounding.
Here is what the letters mean:
T = terra (earth)
N = neutral (the neutral conductor of the power system)
I = impedance (value not specified)
C = combined S = separate
Terra (or earth) means, literally, the body of the earth. For the purposes of this discussion, it means an electrical connection to the earth by means of a ground rod buried in the earth.
Neutral means the neutral conductor of the power system. There are two definitions. Conventionally, the neutral conductor is the common point of a three-phase, four-wire (“wye”) bulk power source. This is the first definition.
In two of the three systems, TN and IT, the neutral conductor is connected to the earth by means of a ground rod. From this, we have a second definition: the neutral conductor is the one connected to ground. This definition is important because it is used to identify the neutral conductor
(In the National Electrical Code, the groundED conductor is also designated as the “identified” conductor. The neutral conductor is “identified” by means of insulation color. In the USA and Canada, the color is white. In Europe, the color is blue.)
Impedance means an impedance is connected in series between the neutral conductor and the ground rod. I have heard that the value of this impedance is from 1000 to 10000 ohms.
Combined means that the function of two conductors is performed by (combined into) one conductor.
Separate means that the function of two conductors is performed (separately) by each of the two conductors.
The power system designations use two letters, TN, TT, and IT. The first letter indicates the means of grounding of the neutral conductor. The second letter indicates the means of grounding of the protective conductor. Now we can define the three major power distribution systems.
TN: The TN system has its neutral connected to a ground rod, and has its protective conductor connected to the neutral. The TN system is the predominant system in the USA and Canada.
The advantage of the TN system is a very low impedance between the protective conductor and the neutral conductor, thus assuring operation of the overcurrent device.
The disadvantage of the TN system is that at, the point of a ground fault, there is a voltage drop across the protective conductor. This raises the potential of accessible grounded parts with respect to the earth, which could result in a shock.
A disadvantage with the USA TN system is that the neutral is grounded at two or more places, one being at the bulk power source and the other at the service entrance. This means that the earth is in parallel with the neutral, and that some of the neutral current will flow in the earth.
In turn, signal grounds between buildings (or even between parts of buildings) can also carry part of the neutral current (which has accounted for fires in some products).
TT: The TT system has its neutral connected to a ground rod, and has its protective conductor connected to its own, separate ground rod. The TT system is the predominant system in the U.K.
The advantage of the TT system is that it overcomes the disadvantage of the TN system. Because the protective conductor has its own, separate earthing, accessible grounded parts of the system are always at earth potential, even in the event of a fault.
The disadvantage of the TT system is that the impedance between the protective conductor and the neutral conductor is not necessarily low, thus compromising operation of the overcurrent device.
IT: The IT system has its neutral connected to an impedance which is connected to ground, and has its protective conductor connected to its own, separate ground rod. (The impedance is 1000 ohms or greater.)
The IT system is the predominant system in France and Norway and other countries.
One advantage of the IT system is that it overcomes the disadvantage of the TN system. Because the protective conductor has its own, separate earthing, accessible grounded parts of the system are always at earth potential, even in the event of a fault.
Another advantage of the IT system is that in the event of an earth fault, the system intentionally remains operational, i.e., the overcurrent device does not operate until the second earth fault. (Often, the system employs an earth-fault monitor such that an alarm is activated when an earth-fault occurs and corrective action can be taken.)
The disadvantage of the IT system is that, when an earth-fault occurs, the voltages with respect to earth change. For example, consider the various voltages with respect to ground in a three phase distribution system having 220 volts phase-to-neutral and 380 volts phase-to-phase in Table 1.
|N – Ground
|A – Ground
|0 (fault to earth)
|B – Ground
|C – Ground
|A – N
|B – N
|C – N
|A – B
|A – C
|B – C
N = Neutral
A = Phase A
B = Phase B
C = Phase C
(Note that the phase-to-neutral and phase-to-phase voltages do not change. Since all equipment is connected either phase-to-neutral or phase to-phase, all equipment continues to operate normally even though the system has a ground fault.)
Let’s now look at the variations of the TN system.
TN-S: The TN-S system has separate neutral and protective conductors throughout the system.
This is the normal system in the USA and Canada.
TN-C: The TN-C system has combined neutral and protective conductors throughout the system.
TN-C-S: The TN-C-S system has part of the system with combined neutral and protective conductors and part of the system with separate neutral and protective conductors.
This is normal for USA households where plug-and-socket connected dryers and ranges have the neutral connected directly to the frame of the dryer or range.
Note that, no matter the system, TN, TT, or IT, the scheme of grounding the neutral largely has no impact on the design of the product.
Some authorities do tend to be concerned about the voltage rating of mains-to-ground-connected components where the equipment is intended to be connected to an IT system. They are concerned about those components being subjected to the higher phase-to-ground voltage occurring during a system ground fault.
Some authorities also tend to be concerned about the magnitude of leakage current where the equipment is intended to be connected to an IT system. Again, they are concerned about the higher phase-to-ground voltage occurring during a system ground fault.
Some authorities tend to be concerned about the electric strength and hi-pot test voltage of the mains circuits where the equipment is intended to be connected to an IT system. Note, however, that the magnitude of transient over-voltages does not necessarily change due to a phase-to-ground fault.
Let’s now turn to polarization. For the purposes of this discussion, polarization is the identification of one or more terminals of a supply system, whether the neutral terminal or a phase terminal. As we have seen, all the conductors of a power distribution system are identified.
For the most part, the neutral conductor — even though it is usually grounded — is treated as if it were a phase conductor.
As mentioned, the TN-C system combines the neutral conductor with the protective conductor. In TN-C systems and equipment, it is essential for safety that polarization be observed, i.e., that the neutral in the equipment be connected to the neutral in the supply system. Consider the home electric dryer which has its metal enclosure connected to the neutral terminal of the supply cord. For prevention of electric shock, it is imperative that the dryer neutral only be connected to the supply neutral. Polarity must be observed.
In the USA, lamps employing Edison-base sockets are required to be provided with a polarized plug. The neutral terminal of the plug is connected to the screwshell of the socket. This means that the screwshell, being accessible, is at ground potential. This improves the safety of the Edison-base socket.
Polarization can be used to improve the safety of equipment where both poles of the supply are not treated equally.
Polarization via socket outlets is not consistent in various power distribution systems. In the US and Canada, polarization is maintained in the 120- volt, 15-amp outlets by one blade being wider than the other. The wider blade is the neutral conductor. (Note that the grounding terminal does not provide the polarization.)
In the U.K., polarization is maintained in the 13-amp socket-outlet by the three L, N, and E blade positions. The wiring is indicated by markings on the plug. Note that two-wire plugs require a dummy grounding terminal for both polarization and for activating the shutters in the socket-outlet.
In Australia and New Zealand, polarization is maintained by the angled blade orientation. The wiring is indicated by markings on the plug.
Polarization is not maintained by the European Schuko, French, Danish, and Swiss socket outlets. Note that the French, Danish, and Swiss plugs can only be inserted one way. But, polarity of the wiring to the socket-outlet is not maintained. Be careful not to assume that just because a plug can only be installed one way in a socket-outlet that it is polarized.
- Ron Wellman of HP Corporate Product Regulations suggested this topic.
- The TN, TT, and IT systems are defined in IEC 364 and repeated in IEC 950.
- For more information on neutral grounding, see the Standard Handbook for Electrical Engineers, Donald G. Fink and H. Wayne Berry, Editors. Published by McGraw-Hill Book Company. ISBN 0-07-020975-8.
Copyright 1995 by Richard Nute Originally published in the Product Safety Newsletter, Vol. 8, No. 5, December 1995
Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations.