ADS

Coby:

How about if I tell you what I do understand?


When I employ EEBADS I know that every conductive part within a building (that is immovable and/or can be gripped by a human hand) will be at the same potential, therefore if any 'hazardous live' line conductor comes into contact with any conductive part within the building, I can predict an automatic disconnection time not exceeding 0.4 seconds.


Therefore; no immovable conductive parts or conductive parts that can be gripped by the human hand, can remain hazardous live for more than 0.4 seconds.


So if you've taken away the "earthed equipotential bonding" element, how does "automatic disconnection of supply" actually work?

OK, that's not quite correct.


The automatic disconnection time is calculated just from the earth fault path of the circuit concerned, without considering earthed equipotential bonding, using the formula:


Z_s = Z_e + (R₁+R₂)


Where:

Z_s the total earth fault loop impedance

Z_e is the external earth fault loop impedance to the origin of the installation (the DNO network)

R₁ is the resistance of the line conductor of the circuit (at 70 degrees)

R₂ is the resistance of the protective conductor of the circuit (at 70 degrees)

Compare Z_s with the relevant Table in Chapter 41 of BS 7671 for the protective device concerned, to make sure you can achieve the disconnection time (usually 0.4 s for installations with TN-C-S or TN-S earthing arrangement, 0.2 s for installations with TT earthing arrangement)



So, what does earthed equipotential bonding do?


When a fault occurs, the point of fault rises in voltage. The voltage rise is due to Ohms Law, and depends on the fault current flowing, and the resistance of the protective conductor path. It could be anywhere up to the full line to neutral voltage of the system (230 V), if the resistance in the earth path is very large compared to the resistance of the line conductors. The voltage is usually greater in TT systems than TN-S or TN-C-S systems. The voltage will last for the duration of the fault (0.4 s in most circuits in installations with TN-C-S or TN-S earthing arrangement, 0.2 s in most circuits in installations with TT earthing arrangement).


If a metal pipe comes into the premises from outdoors, this pipe will be at the same potential as the neutral of the supply transformer.


We use equipotential bonding to help reduce the touch voltage between the metal parts of the equipment with a fault, and the metal pipes from outdoors, for the duration of the fault (0.4 s in most circuits in installations with TN-C-S or TN-S earthing arrangement, 0.2 s in most circuits in installations with TT earthing arrangement). SO, really equipotential bonding has nothing to do with the circuit, but metal parts connected outside the installation.


Equipotential bonding does not make the touch voltage zero, but often:

 - it will be much lower, inside houses with TT earthing arrangements, than inside houses with TN-S or TN-C-S earthing arrangements

 - It will be much higher outside houses (in the garden) with TT earthing arrangements, than outside houses with TN-S or TN-C-S earthing arrangements.

Coby:

How about if I tell you what I do understand?


When I employ EEBADS I know that every conductive part within a building (that is immovable and/or can be gripped by a human hand) will be at the same potential, therefore if any 'hazardous live' line conductor comes into contact with any conductive part within the building, I can predict an automatic disconnection time not exceeding 0.4 seconds.


Therefore; no immovable conductive parts or conductive parts that can be gripped by the human hand, can remain hazardous live for more than 0.4 seconds.


So if you've taken away the "earthed equipotential bonding" element, how does "automatic disconnection of supply" actually work?

OK, that's not quite correct.


The automatic disconnection time is calculated just from the earth fault path of the circuit concerned, without considering earthed equipotential bonding, using the formula:


Z_s = Z_e + (R₁+R₂)


Where:

Z_s the total earth fault loop impedance

Z_e is the external earth fault loop impedance to the origin of the installation (the DNO network)

R₁ is the resistance of the line conductor of the circuit (at 70 degrees)

R₂ is the resistance of the protective conductor of the circuit (at 70 degrees)

Compare Z_s with the relevant Table in Chapter 41 of BS 7671 for the protective device concerned, to make sure you can achieve the disconnection time (usually 0.4 s for installations with TN-C-S or TN-S earthing arrangement, 0.2 s for installations with TT earthing arrangement)



So, what does earthed equipotential bonding do?


When a fault occurs, the point of fault rises in voltage. The voltage rise is due to Ohms Law, and depends on the fault current flowing, and the resistance of the protective conductor path. It could be anywhere up to the full line to neutral voltage of the system (230 V), if the resistance in the earth path is very large compared to the resistance of the line conductors. The voltage is usually greater in TT systems than TN-S or TN-C-S systems. The voltage will last for the duration of the fault (0.4 s in most circuits in installations with TN-C-S or TN-S earthing arrangement, 0.2 s in most circuits in installations with TT earthing arrangement).


If a metal pipe comes into the premises from outdoors, this pipe will be at the same potential as the neutral of the supply transformer.


We use equipotential bonding to help reduce the touch voltage between the metal parts of the equipment with a fault, and the metal pipes from outdoors, for the duration of the fault (0.4 s in most circuits in installations with TN-C-S or TN-S earthing arrangement, 0.2 s in most circuits in installations with TT earthing arrangement). SO, really equipotential bonding has nothing to do with the circuit, but metal parts connected outside the installation.


Equipotential bonding does not make the touch voltage zero, but often:

 - it will be much lower, inside houses with TT earthing arrangements, than inside houses with TN-S or TN-C-S earthing arrangements

 - It will be much higher outside houses (in the garden) with TT earthing arrangements, than outside houses with TN-S or TN-C-S earthing arrangements.