Talk:Earthing and Bonding

I took out the following since they should probably go in the article on types of earthing schemes. This article is about clarifying what is earthing and what is bonding

Some premises are not provided with an earth - these are typically farms with livestock (or stabling for horses), petrol stations, and temporary supplies.

In some cases it may not be possible for the REC to provide an earth as it may change the line characteristics (if overhead fed), your supply may not be capable of supporting an earth (TNS) where you have no suitable conductor, or where the network is otherwise unsuitable for various reasons, i.e. areas subject to tidal influence, tide comes in = good earth, tide goes out = bad earth.

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To-Do: incorporate changes suggested by Andy Wade

   On Mon, 07 Jul 2008 00:58:02 +0100, Andy Wade wrote:
   
   > YAPH wrote:
   > 
   >> Earthing ensures that in the event of a fault a large fault current will
   >> flow causing rapid operation of a Circuit Protective Device
   > 
   > The fault current doesn't necessarily have to be 'large'.  In principle 
   > 30 mA will do the trick if the device doing the automatic disconnection 
   > is an RCD.
   > 
   >> [...] (fuse or circuit breaker) promptly disconnecting the supply.
   > 
   > Or RCD in some circumstances.  (OK "circuit breaker" in its most general 
   > sense includes RCCBs, but saying "fuse, MCB or RCD" might be clearer for 
   > most readers.)
   > 
   >> Earthing is also designed to limit the voltage rise of anything earthed
   >> that one can touch to a safe level (specified as being under 50V AC)
   >> during the fault.
   > 
   > That's wrong.  Touch voltage can be much higher during an earth fault: 
   > half-mains or more, due to the voltage drop in the CPC.  Shock 
   > protection is achieved by limiting the fault duration, not the fault 
   > voltage.  Only in particular cases is it necessary to provide local 
   > supplementary bonding to limit the voltage too.
   > 
   >> The size of earthing conductors and characteristics of fuses and other
   >> protective devices are designed to ensure that a fault is cleared within 5
   >> seconds for all faults, and within 0.4 seconds for circuits feeding socket
   >> outlets (to which our washing machines etc, which may be touched, are
   >> connected).
   > 
   > This is out of date information now.  Permissible disconnection times 
   > have changed significantly in the 17th ed. and the old distinction 
   > between socket circuits and fixed equipment has gone.  For 230 V AC 
   > supplies (incl. 230/400 V 3-ph) the following maxima now apply:
   > 
   >   System                     TN     TT
   >                            -----  -----
   >   Final circuits <= 32 A   0.4 s  0.2 s
   >   Final circuits > 32 A      5 s    1 s
   >   Distribution circuits      5 s    1 s
   > 
   > 
   > Remember the mantra: earthing protects by reducing the duration of touch 
   > voltages.  Bonding protects by reducing their magnitude.
   >