SAFE EARTHING SYSTEMS
Before we can discuss the problem of how to provide adequate and safe earthing systems, we should first consider the causes of earth faults. These can be caused by Lightning, Earth Leakage Current & Voltage, Differential Voltage Faults, Cable Capacitance Discharge, Circulating Currents, Fault Conditions from Loose or Inadequate Connections, or Human (Installation) errors, including the absence of the correct electrical protection devices, the installation of incorrect protection devices, incorrect installation, or failure to observe regulations. Good quality and reliable earthing systems are therefore essential to protect against the situations arising as previously mentioned.
Both cable manufacturers and cable gland manufacturers must cater in their design for the eventuality of an earth fault being carried via the cable armour, which is primarily intended for mechanical protection, but which may also be able to double up as the safe means to carry the earth fault conditions to ground. This may be the case with some Low Voltage cables as well as Medium & High Voltage cables, but in the latter two cases a higher fault current may be expected to occur in these cables.
Each cable that is expected to be able to carry an earth fault via its integral armour would have a rated current carrying capacity. When a metallic cable gland is connected to an equipment enclosure which may also be metallic, or alternatively non-metallic, an Earth Tag may be required to ground the earth circuit via an Earth Link Cable or connect to another piece of equipment. These are intended to provide an earth bond connection as specified in BS 6121 : Part 5 : 1993, and subsequently EN 50262.
Here are the Short Circuit Fault Current Rating values for CMP slip on Earth Tags, when installed between the cable gland and equipment :-
|CMP Earth Tag Size||Short Circuit Rating Symmetrical Fault Current (kA) for 1 second|
CMP CIEL Cable Glands
In the event that MV and HV cables have a High Fault Current Carrying Capacity, via their Single Wire Armour configurations, the fault rating of the cable may be higher than those of the standard CMP Earth Tags. In this case it would not be advisable to use standard earth tags as it is important to match the rated fault current of the earth tag to the corresponding fault current rating of the cable armour. If the rating of the earth tag is greater than the fault current rating of the mating cable armour then using the earth tag would be acceptable, but the situation may be unsafe if the rating of the cable armour fault current carrying capacity is greater than that of the standard earth tag.
In such cases the perfect solution to overcome potential over heating and possible flash over, would be to use the CMP CIEL solution which involves the inclusion of an integral Cast Integral Earth Lug arrangement, in the cable gland assembly. The CMP CIEL range uses the same principal design concept as a standard CW, E1W, or E2W cable gland product in terms of sealing and / or Explosive Atmosphere protection, but with additional facility for extra heavy duty earth connection.
The following fault current ratings apply to the CMP CIEL design type cable glands.
|CMP CIEL Gland Size||Short Circuit Ratings Symmetrical Fault Current (kA) for 1 second|
|20S to 40||26.0|
|50S to 90||43.0|
In addition to the recognised causes of earth faults described above, other environmental factors would also have an impact upon the overall electrical performance, and require reliable safe earthing systems in order to protect against them. These environmental considerations would include the possibility of Electromagnetic Interference (EMI / RFI), cross talk and induced voltages stemming from the positioning of cables.
It has been a known fact for many years in the UK power generation industry that unless special measures are taken to both segregate the cables in accordance with wiring regulations, and also isolate the metallic cable armour from running in continuous contact throughout the whole of the earthing system, interference or electrical noise was likely to spread around the extent of the plant.
To combat the sometimes intense levels of interference that had been experienced in practice, users were able to use the CMP ZEN range of Insulated Cable Glands (B323/B347, B324/B348, B327/B350 & B367/B368), which would allow the cables to be earthed at one end only, for example the supply end, and insulated at the field end. ZEN is available with an insulating arrangement to ensure that no contact is made between the earth circuit carried through the cable armour and the electrical enclosure which the cable enters. This isolation of the cable armour at one end of the cable is sometimes referred to as single point earthing.
The ZEN range of products from CMP however goes much further than merely single point earthing, and allows great flexibility in the earth system, delivering optimum safety into the bargain. The CMP ZEN cable gland is available with or without the integral extra heavy duty earth lug featured in the CMP CIEL cable gland range, and this allows a whole range of options to be considered in terms of engineering solutions being adopted. Either in conjunction with other cable glands from the CMP CIEL family or not, the CMP ZEN product enables engineers to design safe earthing systems that can effectively deal with :-
- Cross Talk Noise Reduction
- The Electrical Separation of Main Earths
- Earth Fault Segregation
- Fault Current Reduction with Series Cable Resistance
- Electrical Noise Reduction in Instrument Cables
- Circulating Current Protection in Single Core Cables or Three Core Cables with unbalanced Load Conditions
It may also be of interest to note that versions of the ZEN cable gland range are available from CMP Products which have been tested approved and used in containment areas of the Nuclear Power Generation industry.