2365 Design Project - May need some advice.

[RDB85]

From my point of view you have specified Type B for the MCBs for all circuits except the Fridges, therefore you need to specify what type MCB you will use for the Fridges.

The Fridges are quite large, it appears; as I said in an earlier post most commercial fridges would not exceed 500W (2.1A). However a fridge, since it is running a compressor motor, when started does have a high peak current. This current might be 10X the normal running current but for a fraction of a second. If you only had one fridge on each 16A radial circuit then the peak current of, say 21A, would not trip the 16A type B circuit breaker. If you had six fridges on one 16A circuit and they were all started simultaneously then it is possible the peak current could trip a 16A type B breaker and you may wish to select a type C breaker to ensure this high starting current did not trip the breaker.
Functional earthing is generally only found in things like PELV systems or RCDs with functional earthing conductors. Functional earths are an earth reference that is not intended to provide a safety function. If the conductor is both functional and protective then the CSA is determined by the protective part, if just functional then by the minimum CSA given in table 52.3 in section 524.
Main protective bonding is covered in section 544.

Thanks for your help on that Richard. Much appreciated.

 

[RDB85]

I have looked at the Main Protective Bonding, Supplementary Bonding, Earthing Conductor and CPC in the regs, I am just wondering if I am on the right lines:

• PME Conditions Apply - 25mm2
• Non PME Applications - 10mm2

Supply Equipotential Bonding:

• Without Mechanical Protection - 16mm2
• With Mechanical Protection - 16mm2

Earthing Conductor:

• TT Installation - 2.5mm2
• TN Installation - 6mm2

CPC:

• Without Mechanical Protection - 4mm2
• With Mechanical Protection - 2.5mm2

 

[Richard Burns]

BS7671 states that every (non protective bonding) protective conductor must be calculated from the adiabatic equation or selected from table 54.7. This is the basic requirement to meet.

Then there are also other limits applied for various circumstances:
For unenclosed single core copper cables: 2.5mm², or 4mm² if not mechanically protected (sheathed)
For buried earthing conductors a lot of different scenarios; the smallest csa permitted being 2.5mm².

For supplementary bonding conductors generally a minimum of 2.5mm², or 4mm² if not mechanically protected (sheathed) however where exposed conductive parts are connected the csa of the cpc(s) of the circuit supplying those parts must be taken into consideration when determining the minimum value.

Then for main protective bonding conductors the csa is determined by table 54.8 and reference to the csa of the incoming supply PEN conductor, where the supply is PME, the minimum csa being 10mm². Note that the earthing conductor on PME supplies also needs to meet these conditions.
Where the supply is not PME then the minimum csa is 6mm² and >= half the minimum required csa of the earthing conductor (not the actual installed csa of the earthing conductor).

Therefore I would suggest that you look again at the values you have given for the main protective bonding conductors.

Consider the practicality of using 16mm² cable for supplementary bonding and review the sizes you have given.

For the earthing conductor the TT size is correct.
Consider the requirements of 542.3.1 when giving the minimum csa for the earthing conductor on a TN system in general. Perhaps provide values for TNS and TNCS separately.

For cpc csa if the minimum is 2.5mm² then how can we use 1.0mm² twin and earth with a cpc size of 1.0mm² and 1.5 - 4 mm² twin and earth with a cpc size of 1.5mm²?
What conditions for the cpc allow this to occur?
The minimum sizes you have given would correctly apply for single core copper cables.