Increased Safety Exe and Circuit Breaker Sizing

[biglee82]

Hi all

I've had the following put to me and can find no reference in relevant parts of BS EN 60079 (elec apparatus for explosive environments) nor a previous standard BS 5345 (selection, installation and maintenance of ...)

We apply a rule of thumb of 4A per mm2 for cables running through potentially contaminated land, which apparently comes from an old pipeline operators code of practice and is recommended to keep temperature cables low. It is also applied to terminals in the hazardous area. Also worth noting is it is associated with Increased Safety Exe equipment.


This therfore causes an issue when re-using cables but replacing the protection and load. The cable in question here is a supply to a lighting column. We have replaced the fittings and fitted an Exe isolator at the column and installed a 10A Type C breaker in accordance with manufacturers instructions. Applying the 4A / mm2 rule to a 1.5mm2 cable allows 6A, The cable is rated to 20A. The expected load is around 4A. Under fault conditons the cable could see over 10A without bothering the breaker thus heating the cable and terminals. Electrically in line with BS7671 the installation is sound.

Any experience with this?? Any thoughts??

 

[7anoter4]

If the cable runs through a steel rigid conduit and the conduit connections are threaded I think you may use it in hazardous area if the conductor temperature will not be more than permissible for the insulation-for instance for PVC 70oC and for XLPE 90oC.Except T6 temperature classification [85oC maximum permissible ] all other material permissible temperature is above 90oC.
In order to calculate the maximum permissible current for a cable running in a conduit you may follow the IEC 60287 way. The maximum permissible current will heat the conductor up to maximum permissible insulation temperature. This depends upon conductor losses[I^2*Rac where Rac is the conductor resistance at maximum temperature[for instance 90oC] in alternative current [50 or 60 Hz].If the conductor cross section is less than 6-10 sqr.mm than Rac=Rdc.
Also depends on how these losses are dissipated in the air-wall arrangement.
Let's say the cable is 3*1.5 copper conductors PVC insulated. The air temperature is 40 degrees C.
Conductor dia. =1.38 mm. insulation core dia= 2.8 mm overall dia=11.6 mm.
3/4" Galvanized Steel Rigid Conduit. One cable per conduit. One conduit in 40 oC open air. Maximum permitted current is 13.4 A.
For 1.5 sqr.mm the current density is 13.4/1.5=8.93 A/mm^2.
Larger cross section lower the current density .So for 240 sqr.mm will be 450/240=1.875 A/mm^2.