Protection against fire due to earth faults
RCDs are very effective devices to provide protection against fire risk due to insulation fault because they can detect leakage currents (ex : 300 mA) which are too low for the other protections, but sufficient to cause a fire.
The great majority of electrical short-circuit in low voltage installation are line to earth insulation failure.
The protective measures against electric shock presented in previous section of this chapter will ensure automatic disconnection of the supply in case of fault between a line conductor and accessible conductive part that could lead to dangerous touch voltages.
But fault between a line conductor and earth with lower amplitude than cable overcurrent protection threshold (and no risk of “indirect contact”) may also happen (see Figure F77).
Protection with RCDs
An insulation failure between line conductor and earth in dusty and humid environment for instance can lead to an arc fault of low intensity according to line conductor withstand, but high enough to start a fire. Some tests have shown that even a fault current as low as 300 mA can induce a real risk of fire (see Figure F78)
This type of fault current is too low to be detected by the overcurrent protection.
For TT, IT and TN-S systems the use of 300 mA sensitivity RCDs provides a good protection against fire risk due to this type of fault. (see Figure F79)
The IEC 60364-4-42:2010 (clause 422.3.9) makes it mandatory to install RCDs of sensitivity ≤ 300 mA in high fire-risk locations (locations with risks of fire due to the nature of processed or stored materials - BE2 condition described in Table 51A of IEC 60364-5-51:2005). TN-C arrangement is also excluded and TN-S must be adopted.
In locations where RCDs are not mandatory as per the IEC, it is still strongly recommended that you consider the use of RCDs, bearing in mind the potential consequences of fire.
See Residual Current Devices (RCDs) for the selection of RCDs.
Another solution is to use ground fault protection (see below) but the range of fault current detected will be reduced.
Protection with “Ground fault protection”
In TN-C system, RCD protection cannot be used, as the measurement of earth fault current by a sensor around line conductors and PEN will lead to permanent wrong measurement and unwanted trip. But a protection less sensitive than RCD but more sensitive than conductors’ overcurrent protection can be proposed. In North America this protection is commonly used and known as “Ground Fault Protection”.
Different types of ground fault protections (GFP)
(see Figure F82)
Three types of GFP may be used, depending on the measuring device installed:
“Residual Sensing” RS
The “insulation fault” current is calculated using the vectorial sum of currents of current transformers secondaries. The current transformer on the neutral conductor is often outside the circuit breaker.
“Source Ground Return” SGR
The “insulation fault current“ is measured in the neutral – earth link of the LV transformer. The current transformer is outside the circuit breaker.
“Zero Sequence“ ZS (Equivalent to IEC RCD in principle)
The “insulation fault“ is directly measured at the secondary of the current transformer using the sum of currents in live conductors. This type of GFPis only used with low fault current values.
Ground fault protection can be included in the circuit breaker (see Figure F81) or performed by a standalone relay. In all cases the device operated by the GFP shall have the breaking capacity of the maximum fault current at the point of installation, alone or in coordination with another overcurrent protective device.
Earth fault monitoring
Increasing the sensitivity of protection system will reduce the risk of fire but can also increase the risk of unexpected tripping on disturbance that are not real fault. (See Sensitivity of RCDs to disturbances). Where balance between sensitivity and continuity of service is challenging, the monitoring of earth fault with no automatic disconnection brings also benefits.
Earth current monitoring and alarming allow:
- Early detection of deterioration of insulation
- Abnormal leakage currents
- Detection of neutral to earth unwanted contact in other place than the dedicated earthing link
- For more information about electrical fire origins, and the latest solutions to mitigate the risks, download the Electrical Fire Prevention Guide (PDF)