Protection against fire due to insulation failure

RCDs with a sensitivity no more than 300mA provide protection against fire risk due to insulation failure 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 a fault between a line conductor and the earth resulting in a fault current lower than the overcurrent protection threshold may also happen (see Figure F74).

Fig. F74 – Overcurrent protection curve and earth fault potential current

How an electrical fire can start due to insulation failure

Insulation failure may lead to tracking currents to the earth. This refers to electrical currents that flow between the lines and the earth on the surface of insulating materials.

Above a certain level, these currents may become dangerous and cause an outbreak of fire in electrical equipment or installation, as explained in Figure F75)

[1] New insulating material has a very high surface resistivity level that prevents the flow of tracking currents.
Over time, pollution on the surface of insulation materials (dust, various types of soiling, etc.) may occur.
If such pollution deposit remains dry, surface resistivity will stay very high, so no risk with tracking currents.

[2] In case of humidity, for example due to condensation, the pollution on the surface of insulation will absorb humidity and become conductive.
A low current may then flow on the surface of the insulation between the lines and the earth.
When the surface of the material dries, micro-arcs may appear locally, which cause deterioration of the insulating material and locally produce carbon deposits, thereby increasing the tracking current.

[3] If the tracking current exceeds 300 mA, surface heating and the sparks produced provide enough energy to set fire to the carbon deposits.

Fig. F75 – Origin of fires in building (insulation failure)

This type of fault current is too low to be detected by the overcurrent protection.

Protection with RCDs

For TT, IT and TN-S systems the use of 300 mA sensitivity RCDs provides protection against fire risk due to insulation failure. (see Figure F76)

Fig. F76 – Example of tripping curve of earth leakage protection

According to IEC 60364‑4‑42:2024 (see clauses 422.3.8 and 426) it is mandatory to install RCD with sensivity no more than 300 mA to protect against fires due to tracking currents to earth.

This applies to the following circuits:

• Circuits supplying locations classified BE2 for risk of fires, due to the nature of processed or stored materials (e.g. barns, wood-working shops, paper factories ...)
• Circuits supplying locations intended for people to sleep (e.g. facility for elderly persons, persons with disabilities, hotels, elementary schools, kindergarten, dwellings)
• Circuits supplying locations with irreplaceable goods (e.g. museums, historical buildings, archives)
• Workplaces in highrise buildings
• Circuits supplying agricultural premises with livestock

In locations where RCDs are not mandatory as per the IEC 60364-4-42, it is still recommended to 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)

Three types of ground fault protections may be used, depending on the measuring device installed:

• Residual Sensing (RS)
• Source Ground Return (SGR)
• Zero Sequence (ZS)

Fig. F77 – Different types of ground fault prodections

• 

  RS system

• 

  SGR system

• 

  ZS system

Ground fault protection can be included in the circuit breaker (see Figure F78b) 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.

“Residual Sensing” RS

The “insulation fault” current is calculated using the vectorial sum of the currents measured by each conductor current transformers secondaries. The current transformer on the neutral conductor is often outside the circuit breaker.

Fig. F78 – Residual Sensing ground fault protection

• 

  [a] Example of tripping curve

• 

  [b] Compact NSX630 with integrated RS ground fault protection Micrologic 6.3E

“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 GFP is only used with low fault current values.

Earth fault monitoring

Refer to State-of-the-art solutions for electrical fire prevention, beyond the standards