Text book explanations don't always make sense. See what you all make of this extract.
From (British publication) The Electrician's Guide to the 16th Edition IEE Wiring Regulations, BS7671
, section 5.4.3, sub-sec. 2, Supplementary bonding conductors:
There will sometimes be doubt if a particular piece of metalwork should be bonded. The answer must always be that bonding will be necessary if there is a danger of severe shock when contact is made between a live system and the metalwork in question. Thus if the resistance between the metalwork and the general mass of earth is low enough to permit the passage of a dangerous shock current, then the metalwork must be bonded.
The question can be resolved by measuring the resistance (Rx) from the metalwork concerned to the main earthing terminal. Using this value in the formula:
Ib = Uo / (Rp + Rx)
will allow calculation of the maximum current likely to pass through the human body where:
Ib is the shock current through the body (A)
Uo is the voltage of the supply (V)
Rp is the resistance of the human body (ohms)
Rx is the measured resistance from the metalwork concerned to the main earthing terminal (ohms).
The resistance of the human body, Rp can in most cases be taken as 1000 ohms although 200 ohms would be a safer value if the metalwork in question can be touched by a person in a bath. Although no hard and fast rules are possible for the value of a safe shock current, Ib, it is probable that 10mA is seldom likely to prove fatal. Using this value with 240V for the supply voltage, Uo, and 1000 ohms as the human body resistance, Rp, the minimum safe value of Rp (sic) calculates to 23k ohms. If the safer values of 5mA for Ib and 200 ohms for Rp are used, the value of Rx would be 47.8k ohms for a 240V supply.
To sum up, when in doubt about the need to bond metalwork, measure its resistance to the main earthing terminal. If this value is 50k ohms or greater, no bonding is necessary. In a situation where a person is not wet, bonding could be ignored where the resistance to the main earthing terminal is as low as 25k ohms.
Looking at the line "when contact is made between a live system and the metalwork in question," my first thought was that this could be interpreted two different ways:
(a) Direct contact between an energized conductor/component and the metalwork, thus energizing that metalwork, OR
(b) Somebody touching an energized circuit/component while being simultaneously in contact with the metalwork.
In case (a) it is obvious that all such metalwork should be bonded to ground to prevent it becoming energized. But the following sentence and the formula which clearly places Rp and Rx in series means that they must be referring to case (b). With me so far? Good!
So, the whole remainder of this section is talking about minimizing the shock risk when somebody simultaneously contacts the exposed metalwork and an enrgized conductor.
They state that if the resistance of the metalwork to ground is over 50K then it doesn't need to be bonded. O.K., so the shock current at 50k (using their own body resistance value of 1k) will be 240V/51k = 4.7mA.
Now let's take a case where Rx is, say, 20k. If left unbonded, the shock current would be 240V/21k = 11.4mA. But by their rules, this metalwork would be bonded to ground, thus reducing the total series resistance to Rp (all but a couple of ohms!) and increasing the current to 240mA.
The rule quoted by this section seems to be saying, in effect: If the shock current with the metal unbonded is under 10mA or so, then we'll leave it at that. But if the shock current would be over 10mA and pose more of a risk, then we'll bond the metal to make sure the sucker gets hit with a full 240mA!
I'm not arguing for or against bonding here; I'm just saying that the explanation in this section makes no sense.