Single versus double-pole circuit breakers

CBI Electric Australia Pty Ltd
By Viv Cohen, Circuit Breaker Industries
Friday, 10 August, 2007


Earth leakage protection and earthing

The principles governing sensitive earth leakage protection and its dependence on the earthing system are examined, together with both electrical shock and fire hazards that could result from elevated potentials on the neutral conductor. Justifications are presented identifying the basic safety requirement to isolate all active conductors including neutral, where assurances cannot be given that the neutral conductor is always reliably maintained at earth potential.

Considering the critical function and importance of electrical safety equipment such as earth leakage circuit breakers (ELCBs), this article examines the installation and application environment and conditions that might influence the final choice of suitable devices.

Earthing, bonding and touch voltages

Problems in the equipment earthing conductor are not easily detected since, under normal operating conditions, no current flows in this conductor. From the time it's installed until the time an earth fault occurs, the quality of the earthing conductor remains an unknown value. In the absence of periodic testing, the long-term efficacy of earthing conductors or the complete earth loop cannot be ensured.

It's also important to understand that AS/NZS 3000:2000 and most international standards don't recognise automatic disconnection of the supply by RCDs as the sole means of protection against indirect contact in the absence of a protective earthing system. In the case of indirect contact, automatic disconnection of supply is intended to limit the prospective touch voltage that could appear between simultaneously accessible conductive parts in the event of a fault between a live part and exposed conductive parts or a protective earthing conductor.

It's generally accepted that, under normal operating conditions, the limit of safe touch voltage is 50. In many cases however, newer information indicates that varying degrees of stress can occur at voltages even lower than 25. For 50 Hz AC, muscular effects have been demonstrated at voltages as low as 7 to 13 in normally damp environments and falling even lower under conditions where perspiration is present. For large contact areas in wet conditions and depending upon current paths, it has been confirmed that ventricular fibrillation can result from touch voltages as low as 25. Several researchers suggest that even more electrical accidents occur due to contact with exposed metalwork rather than through direct contact with electrical conductors.

Where 30 mA sensitive ELCBs are installed, one would expect reliable operation for earth loop impedances of up to 8000Ω (240 V/30 mA). This indicates that even in conditions where high earth loop impedances are prevalent, protection against direct contact with the mains supply would be ensured. However in conditions of indirect contact, these same assurances may not be applicable unless total isolation is ensured, including that of the neutral conductor.

Even in solidly earthed systems, neutral elevation can occur due to:

  • imperfect earth bond connections
  • voltage drop in the neutral conductor due to single phase loads
  • voltage drop in the neutral conductor resulting from harmonic currents
  • back emf from single-phase motors after being switched from the supply mains.

The immediate conclusion is that safety against shock hazard cannot be ensured even when 30 mA sensitive ELCBs are used, unless every effort is made to remove all sources of voltage from the victim, including voltages that might appear on the neutral conductor. This implies the use of ELCBs that are capable of interrupting all active conductors, including neutral. The obvious reason for this is that should there exist any neutral potential above earth, extraneous conductive parts could remain 'live' even after a one-pole breaking RCBO had tripped.

Safety isolation

Safety isolation indicates a situation where all power frequency voltage from all active conductors, including neutral, is removed from the circuit once the isolation device is tripped or switched to the 'off' position. For RCBOs in which the neutral pole is not switched, the term 'isolation' can only be understood to apply in the strict sense of the word if the RCBO is installed in an electrical circuit in which the neutral can be considered to be reliably maintained at earth potential. While it is not unreasonable to assume such a situation in a residential type installation where the MEN link is made at the main switchboard, it doesn't necessarily follow that this same assumption can be applied to large industrial and mining installations.

Fire hazards

Electrical fires associated with overheating from overloading of the installation or electrical equipment connected to it should, in normal circumstances, be prevented through detection of the overload by the installed overcurrent protective devices. If, however, the impedance of the fault restricts the resultant current to less than the rated current of the circuit, conventional overcurrent protective devices wouldn't detect such a fault. Hence, a low current that's arcing or tracking between conductors presents a different problem. Since such faults will invariably involve some flow of current to earth, the use of sensitive earth leakage protection devices can make a significant contribution to eliminating fire risks.

In many countries, statistical evidence indicates that almost 50% of all fires are associated with electrical faults. Earth fault currents have been shown to be a major factor in such incidents, since fault currents flowing through imperfect earth return paths for extended time periods are capable of initiating incendiary ignition. It has been demonstrated that currents as low as 300 mA in imperfect earth paths, given enough time, can result in incendiary ignition.

The latest IEC requirement states that in both TN and TT systems where it's necessary to limit the consequence of fault currents in wiring systems from the point of view of fire risk, the circuits shall be protected by RCDs that have a rated residual operating current not exceeding 300 mA and switch the neutral conductor.

Restricted earth fault installations

For the TN distribution system, in general, the neutral conductor from the transformer is, by definition, directly connected to earth, resulting in a highly stable arrangement through the creation of a 'vectorial' reference point for the neutral. The downside of such systems where the neutral is solidly grounded is that short circuit fault currents are high. It's therefore not uncommon, particularly in mining installations, to restrict the levels of short circuit current by inserting a suitable impedance between the star-point of the transformer and earth. Depending on the type of protection used, the degree to which short circuit fault currents are restricted may vary from as low as a few amps up to hundreds of amps. On typical low voltage systems, this equates to the use of limiting impedance values of some ohms up to tens of ohms.

In restricted earth fault installations, the earth fault currents that pass through the neutral earthing impedance will result in the transformer star-point connection and hence the neutral conductor being elevated above earth potential for the duration of the fault current. When considering possible electrical shock hazards, the levels of neutral voltage elevation are significant even with the use of relatively low ohmic values of impedance in achieving the earth fault restriction.

Earth leakage protection with alternative supplies

Special installation requirements may be necessary for installations that include alternative supply arrangements eg, where a generating set may operate in parallel with the normal supply. In this case, it's essential to ensure that incorrect operation of earth leakage protection devices due to the existence of any parallel neutral to earth path is avoided. Such incorrect operation can be avoided by the use of RCBOs that include switching of both active and neutral conductors.

Polarity reversal

In normal circumstances, and having competent personnel responsible for the construction and inspection of electrical installations, the possibilities of polarity reversal of electrical conductors might be considered relatively rare. Such occurrences cannot be ignored however. For the specific case of single phase RCBOs, such polarity reversal could result from inadvertent errors not only in connecting the supply conductors to the device itself, but also to upstream polarity reversals in the network connections. Polarity reversal in single phase supplies will immediately result in the neutral conductor rising to the full phase voltage. It is only the switching and isolation of both the active conductor and the neutral conductor that can ensure operator safety under such circumstances.

Conclusions

This article has identified the need to select earth leakage protection circuit breakers in consideration of their dependence on the network or system into which they may be installed or applied. It must be conceded that the achievement of total safety may in most circumstances prove to be a relatively unattainable ideal. However, for those cases where equipment has been developed for the specific purpose of providing a safety function, it is vital that every possible effort is expended in attempting to at least approach that ideal.

For indirect contact shock hazard protection, it has been identified that neither national nor international safety standards recognise the use of automatic disconnection from the electricity supply in the absence of the provision of an efficient earthing system. The importance of this requirement has been confirmed through examples given to demonstrate the probability and consequences of neutral potential rise, even in electricity distribution networks and installations in which it is assumed that the neutral conductor is solidly connected to earth.

In addition to the protection against shock hazard, the value of earth leakage protection circuit breakers in the mitigation against fire hazard is now beginning to be understood.

With both shock hazard protection and overcurrent protection facilities being available in a single device, sometimes referred to as an RCBO, the cost effectiveness of these devices is improving. For mainly cost-driven reasons, RCBO options are available where only the active pole and not the neutral pole is switched. This option is specifically intended for installation in networks where the neutral conductor can be considered to be reliably connected to earth.

It is imperative that for applications where it cannot be guaranteed that, for power frequencies, the neutral conductor is always reliably maintained at earth potential, in the interests of safety all active conductors, including the neutral, are switched.

These RCBOs include multiple levels of protection:

  • overload protection
  • short circuit protection
  • shock hazard protection
  • mitigation against fire hazard
  • effective safety isolation through switching of all active conductors.
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