The introduction to Earthing System in Oil & Gas Installations was previously discussed in this article. This article provides further explaination on the advantages and disadvantages of various earthing systems.
Earthing Arrangements
IEC 60364 distinguishes 3 types of earthing arrangements using the two letter codes (i.e. TN, TT and IT). While the first letter indicates the connection between earth and the power supply equipment (generator or transformer), the second letter indicates the connection between earth and the electrical device being supplied on the consumer side. Refer below notations:
T : Direct connection of a point with earth
I : Isolated, or no point is connected with earth
N : Direct connection to Neutral
| Parameter | TT | IT | TNS | TNC | TNCS |
|---|---|---|---|---|---|
| Earth fault-loop impedance | High | Highest | Low | Low | Low |
| Use of RCD is preferred | Yes | No | Yes | No | No |
| Earth electrode required on-site | Yes | Yes | No | No | No |
| Cost of PE Conductor | Low | Low | Highest | Least | High |
| Safety Risk | High loop impedance | Double fault, overvoltage | Broken PE | Broken neutral | Broken neutral |
| Problem with EMI | Least | Least | Low | High | Low |
| Advantages | Safety and reliability | Continuity of operation, low cost | Safest | Low cost | Safety and low cost |
Why TN-C Earthing System is not Recommended?
- Faults at a high voltage level may migrate into the low voltage grid grounding causing touch voltages at low voltage customers.
- A fault in the low voltage network may cause touch voltages at other low voltage customers.
- The potential risk of exposed conductive parts with the neutral conductor in the event of a break-down of the neutral network conductor as well as for low voltage network phase to neutral and phase to ground faults and medium voltage to low voltage faults.
- TN-C system is less effective for Electromagnetic Compatibility (EMC) problems.
- TN-C earthing method is the least safe among earthing methods.
This type of earthing is not commonly used in some countries due to the risks associated with fire in hazardous environments and due to the presence of harmonic currents making it unsuitable for electronic equipment.
In a TN-C system, the RCD devices are quite vulnerable to unwanted triggering from contact between earth conductors of circuits on different RCDs or with real ground. If the threshold settings of the RCDs are increased, then they are far less likely to detect an insulation fault. It is also unsafe to isolate the neutral core in a TN-C system, and hence the RCDs should be wired to interrupt only the live conductor and not the neutral, a convention that is not followed.
In the event of a neutral break, in a single phase system, all parts of the earthing system beyond the neutral break would rise to the potential of the live conductor. In case of an unbalanced three phase system, the potential of the earthing system would move towards the most loaded live conductor. The use of TN-C or TN-C-S system should be banned in such premises. For instance, in the UK, the use of TN-C-S is banned while TT system is recommended for outdoor wiring.