I've been having a large discussion about equipotential bonding and also the common misuse of the term bonding for protective earths.
In essence, as many of you will know, if it's sized to the active and installed for a fault then it's probably not a "bond" as people say but actually a PEC.
Size is not often of importance in equipotential bonding as a conductor of any size, provided negligible impedance will keep the items at the same potential so no risk of shock between touching each, hence the limited required situations in 3000 and them being to do with parts in contact with earth at a different location to our electrode where, depending on location, the different parts of earth that each connect to could rise in potential and form a difference between them.
This has brought up some things that have confused me reading 5.6.3.2.
"The equipotential bonding conductor need not be larger than these sizes
provided that the installation conditions are such that mechanical damage is
unlikely to occur and, in accordance with Clause 5.7.5, a larger size is not
required to reduce the earth fault-loop impedance."
Now, totally understand the minimum is for sufficient mechanical durability and that any smaller size could still equalize potential. It also makes sense to me why a larger conductor probably isn't worth it for the same reason, because the 4mm or 6mm would equalize potential all the same.
However, the there are two other comments there and I have a question around each.
Q1: It somewhat makes sense that a larger size could once again be used for mechanical durability, much like how a 4mm is for the minimum mechanical durability, but wouldn't you sooner just actually provide suitable mechanical protection and still use the minimum size? Rather than simply beefing up the equipotential bond size? Or is it simply indicating either as suitable based on situation and cost
Q2: what does equipotential bonding have to do with EFL. Wouldn't EFL be strictly associated with PECs, if it's part of the fault loop and carrying fault current is it not then a PEC? Or is it something to do with the fact that EQB conductors could end up in parallel with PEC as part of there installation and hence needing to be of sufficient size to carry their portion of the fault current? Which could be high if they also provide another path of low impedance? Also trying to understand how additional EQB can be used to bring down EFLI, wouldn't they actually be PEC and not EQB if they are being used to bring down the impedance of the fault path and hence be a suitable size for this as you would with PEC? I can tell there is some form of link but also something is throwing me off and I'm missing something.
Equipotential bonding and earth fault loop
Re: Equipotential bonding and earth fault loop
I'll tackle those questions in reverse order; but first , a clarification
(apologies if this come across as teaching to suck eggs, but we need to atrt at the beginning).
Correct that the terms "bonding" and earthing" are often used indiscriminately.
Correct that generally "bonding" is short for "equipotential bonding" (EB), and "earthing" is short for "protective earthing" (PE);
and correct that a distinction can be made between them.
Worth noting that,"3000" historically has not always made this distinction correctly or carefully; though 2018 edition is improved from 2007.
The primary function of a PEC is (earth fault) protection; generally using the method known as "automatic disconnection of supply",
in which earth fault current flowing in the PEC is high enough to operate a disconnection device within the maximum permitted time.
The primary function of an EBC is , as you say, maintaining various points at closely similar potential.
However the distinction is between the primary function only.
In most, but not all, cases the conductor in question will be serving both functions (and also possibly others).
When you get right down to it, every PEC is also an EBC.
That's because , when it's doing it's primary job of carrying earth fault current, its own impedance causes potential difference along its length.
So there's a secondary function of keeping everything that'd earthed by that PEC at closely similar potential.
So right there is one reason that EFL comes into "bonding".
The MEC of an installation is not a PEC.
The MEC is there primarily to hold the distribution PEN conductor at close to 0 V to mass of earth.
Each by itself does very little, but having many such connections in parallel does the job.
The MEC in most cases is not capable of performing the required (earth fault) PE function.
Fault protection relies on the fault current flowing from PEC via MEN, to mains PEN, to distribution PEN, back to source.
SFA of it goes via the MEC & electrode.
So little that, even if the low-impedance path via MEN & PENs is absent (as with faulty mains "N"),
not enough current will flow to operate an over-current device within required time
- in these circumstances, fault protection is not provided
So the function of the MEC is primarily an EB function.
Likewise the various cases where EB is required by clause 5.6 are not there to provide PE, only EB.
For these, the source of the potential we are protecting against is external of the installation;
generally being cases where the ground itself can be live (ie at higher potential) to the installation's earthing system.
A different situation is EB on secondary side of transformer providing separated supply.
This type of EBC does not provide (earth) fault protection; as the fault protection is being provided by the fact of being a separated supply.
But it does provide an EB function.
True that in event of 2nd 'earth' fault, (and depending on exact circumstances) can cause disconnection of supply;
but this is not a protective earthing function, it's simply to avoid simultaneously accessible parts being at significantly different potential.
As for Q1, choice of whether to provide mechanical protection or a larger EBC is up to designer, and will depend on circumstances.
Note that in hazardous areas, the minimum size of EBC is often much > 4 mm2.
It all depends of what risks we are protecting against, and what level of potential difference is considered too much.
The downside of EB is that, by connecting a bunch of different metals together, it can facilitate galvanic corrosion.
The trick then is to control the corrosion by providing a sacrificial anode.
Most common in marine & similar environments.
This is the reason it's not advisable to connect a bunch of boats directly to 'shore power'.
Instead we need to introduce a means of blocking DC currents generated by the tiny voltages between various metals,
while still allowing AC earth fault current to flow.
(apologies if this come across as teaching to suck eggs, but we need to atrt at the beginning).
Correct that the terms "bonding" and earthing" are often used indiscriminately.
Correct that generally "bonding" is short for "equipotential bonding" (EB), and "earthing" is short for "protective earthing" (PE);
and correct that a distinction can be made between them.
Worth noting that,"3000" historically has not always made this distinction correctly or carefully; though 2018 edition is improved from 2007.
The primary function of a PEC is (earth fault) protection; generally using the method known as "automatic disconnection of supply",
in which earth fault current flowing in the PEC is high enough to operate a disconnection device within the maximum permitted time.
The primary function of an EBC is , as you say, maintaining various points at closely similar potential.
However the distinction is between the primary function only.
In most, but not all, cases the conductor in question will be serving both functions (and also possibly others).
When you get right down to it, every PEC is also an EBC.
That's because , when it's doing it's primary job of carrying earth fault current, its own impedance causes potential difference along its length.
So there's a secondary function of keeping everything that'd earthed by that PEC at closely similar potential.
So right there is one reason that EFL comes into "bonding".
The MEC of an installation is not a PEC.
The MEC is there primarily to hold the distribution PEN conductor at close to 0 V to mass of earth.
Each by itself does very little, but having many such connections in parallel does the job.
The MEC in most cases is not capable of performing the required (earth fault) PE function.
Fault protection relies on the fault current flowing from PEC via MEN, to mains PEN, to distribution PEN, back to source.
SFA of it goes via the MEC & electrode.
So little that, even if the low-impedance path via MEN & PENs is absent (as with faulty mains "N"),
not enough current will flow to operate an over-current device within required time
- in these circumstances, fault protection is not provided
So the function of the MEC is primarily an EB function.
Likewise the various cases where EB is required by clause 5.6 are not there to provide PE, only EB.
For these, the source of the potential we are protecting against is external of the installation;
generally being cases where the ground itself can be live (ie at higher potential) to the installation's earthing system.
A different situation is EB on secondary side of transformer providing separated supply.
This type of EBC does not provide (earth) fault protection; as the fault protection is being provided by the fact of being a separated supply.
But it does provide an EB function.
True that in event of 2nd 'earth' fault, (and depending on exact circumstances) can cause disconnection of supply;
but this is not a protective earthing function, it's simply to avoid simultaneously accessible parts being at significantly different potential.
As for Q1, choice of whether to provide mechanical protection or a larger EBC is up to designer, and will depend on circumstances.
Note that in hazardous areas, the minimum size of EBC is often much > 4 mm2.
It all depends of what risks we are protecting against, and what level of potential difference is considered too much.
The downside of EB is that, by connecting a bunch of different metals together, it can facilitate galvanic corrosion.
The trick then is to control the corrosion by providing a sacrificial anode.
Most common in marine & similar environments.
This is the reason it's not advisable to connect a bunch of boats directly to 'shore power'.
Instead we need to introduce a means of blocking DC currents generated by the tiny voltages between various metals,
while still allowing AC earth fault current to flow.
