An earthing question

[Farmelectrics]

Put on the spot by my keen nephew today he's an apprentice working on local council re wires and up grades he asks me why does the the bonding to the water have to be in 10mm I say cos the regs say so lol and then try to explain that the lower the reading the better the fault path , but he says well if it's earthed its earthed and you wouldn't get a belt I struggled to explain it any better someone give me a better version so he doesn't think his uncle is a dumbo ha ha

 

[marcspark]

Could say to him,
The purpose of protective bonding is that during an electrical fault where there is a line conductor in contact with an exposed conductive part, that sufficient fault current flows to operate the disconnection device. This will be a fuse, RCB or circuit breaker. It is also there to reduce the touch voltages between exposed conductive parts in the event of a fault on the installation
)

 

[Simonslimline]

Sounds a bonding question to me relating to PME systems.

What happens on this system if the PEN conductor breaks continuity on the suppliers side?

 

[Leesparkykent]

Except where PME conditions apply and for main bonding to lightning protective systems, the main protective bonding conductors must have a CSA of not less than half the CSA of that required of the earthing conductor of the installation, with a minimum size of 6mm2. The CSA need not exceed 25mm2 for copper or the equivalent for other metals. Where PME conditions do apply the main protective bonding conductors must be sized in accordance with the supply neutral conductor and be selected in accordance with Table 54.8.

GN8, section 5.2.4, says "the minimum CSAs given in table 54.8 of BS7671 were intended to obviate the likelihood of protective bonding conductors overheating due to diverted load currents in the event of an open PEN conductor in the PME supply".

Some peoples opinion is that it has been determined to allow the earthing and bonding conductors to withstand both diverted neutral currents and installation earth faults.

 

[marcspark]

Although TT can be a right pain in the butt (trying to get Ra as low as Possible and hammering the stake-s), this is when it's way safer than PME...If you lose Your Neutral or something happens at substation ,loose your earth and still have a live supply. which clearly isn't safe...
I was talking to UK power networks chap, as they were upgrading all the overhead lines to PME. Said they use a earth stake every 4 - 5 poles and was much safer than TT
I said what happens when local farmer with his grass/hedge cutter damages them... Not that safe then...)

 

[Risteard]

The purpose of protective bonding is that during an electrical fault where there is a line conductor in contact with an exposed conductive part, that sufficient fault current flows to operate the disconnection device.

No - that is the function of protective earthing, not bonding.

 

[telectrix]

yep.the purpose of bonding is to ensure as low as possible the touch voltage between an exposed conductive part and an extraneous part for the duration of a fault.

 

[marcspark]

Ops sorry ( must remember to engage brain before fingers get carried away )