Guest
This question relates to earthing systems.
And considers the following scenario –
A substation (say - 11kV/415V) is in an industrial plant and has (contained in a switch yard); an 11kV incoming feed, an 11kV switchgear panel, an 11kV/415V (Dyn11) transformer, and a subsequent 415V, 3 phase and neutral feed to a local process area MCC.
Now consider the scenario where you have within the switch yard, a main earth bar. And bonded to this earth bar (by insulated earth cables) is the following;
-the frame of the 11kV switchgear;
-the frame of the transformer,
-the neutral of the 415V system,
-the fence of the switch yard,
-the un-insulated earth grid or mat around/under the switchyard.
Now consider a fault connection between the frame of the 11kV switchgear and one of the phases of the incoming 11kV supply. In practice, the earth system will then be connected to the 11kV system, including the frames of switchgear, transformer, switchyard fence, earth grid etc. The potential rise of this system could then be to several thousand Volts relative to ‘true’ earth.
My understanding of the situation is now as follows;
The protection system clearance time upstream of the substation will normally not clear in a short time (typically anywhere between 5 seconds to 5 minutes, dependant on the system). The inherent personal protection of the system therefore relies on the principle that the ‘touch’ voltage between frames will be negligible. That is, if you were to touch the 11kV switchgear frame and the fence or transformer frame (at the same time) only a small voltage potential would exist between your two hands. It is also my understanding that if you were to only touch the 11kV switchgear frame (or fence or transformer frame), then there would also be only (relatively) a small voltage potential between your hand(s) and your feet(s). Since the ground potential rise in the earth grid will raise the voltage potential under your feet to a level approaching that of the fence or switchgear frame. It is also my understanding that if you were touching nothing at all the ‘step’ potential between your feet will be a small voltage figure since the ground potential rise (although not uniform) will not significantly vary between your feet due to the earth grid. [In theory – this depends on the effectiveness of the earth grid model, variations in soil resistivity values etc].
The inherent equipment protection would rely on the thermal effects of the fault connection being less than what mechanically could be withstood by any of the equipment.
The first question is – is this ok - does everyone concur so far?
My main question is as follows - what happens to a ‘low voltage’ earth system external to the substation (at the MCC for instance), for the following situations;
1)no low impedance connection between the ‘low voltage’ earth system (external to the substation) and the earth system of the substation,
2)the situation where cable tray or cable ducts from the transformer (for the LV distribution cable) are bonded (purposely or otherwise) to the substation earth system and are then bonded (purposely or otherwise) to the ‘low voltage’ earth system (external to the substation),
My understanding of the situation would then be as follows;
For 1) ‘Transfer’ of voltages between the systems is unlikely to occur. That is, should there be a fault as described above, the 11kV conductor (6350V relative to ‘true’ earth under normal conditions) is not likely to transfer its potential to a remote system away from the substation.
For 2) ‘Transfer’ of voltages is likely. That is, potential rise of the earth system will occur on external equipment such as; MCC frame, MCC main earth, MCC earth grid, domestic earth connections for bonding between MCC and metallic objects such as water pipes, sinks etc.
Now again in this situation, for ‘touch’ voltages between frames, that is, hand-hand contact between two frames that are directly bonded, the potential is small (they are approximately at the same potential, bonding resistance being small). However, the ‘touch’ voltage for the situation where someone is touching an ‘energised’ earth frame and has their feet in contact with the ground (hand-foot path), may now be high. Since the ground under the person’s foot in a ‘domestic’ type situation in an industrial plant will not normally be well connected to the main earth system (no thorough earth grid under foot). [The basic theory behind ‘domestic’ earthing (as I understand it) is to prevent ‘touch’ potentials between conducting objects during a fault on the 415V system, and also for ‘improvement’ of the earth system resistance to earth. If the 415V faults can be cleared quick enough (by the main fault current protection equipment looking at the main fault currents) to prevent leakage currents hand-foot causing physiological problems, then this is deemed ok (the long term effect of circulating ground currents on domestic piping not withstanding). For this discussion, however, the fault cannot be cleared by the 415V board].
This situation practically may arise in ‘unit sub’ arrangements where transformers are physically incorporated into LV MCC panels and the MCC panel has bonded ‘domestic’ type earth connections, and a fault occurs between the 11kV connections of the transformer and the transformer frame, and hence to all other bonded connections.
Would you consider point 2) to be unsafe? Would you consider installations of type 2) common or uncommon? Additionally what is the effect on the neutral (and LV phases) in all these cases?
And considers the following scenario –
A substation (say - 11kV/415V) is in an industrial plant and has (contained in a switch yard); an 11kV incoming feed, an 11kV switchgear panel, an 11kV/415V (Dyn11) transformer, and a subsequent 415V, 3 phase and neutral feed to a local process area MCC.
Now consider the scenario where you have within the switch yard, a main earth bar. And bonded to this earth bar (by insulated earth cables) is the following;
-the frame of the 11kV switchgear;
-the frame of the transformer,
-the neutral of the 415V system,
-the fence of the switch yard,
-the un-insulated earth grid or mat around/under the switchyard.
Now consider a fault connection between the frame of the 11kV switchgear and one of the phases of the incoming 11kV supply. In practice, the earth system will then be connected to the 11kV system, including the frames of switchgear, transformer, switchyard fence, earth grid etc. The potential rise of this system could then be to several thousand Volts relative to ‘true’ earth.
My understanding of the situation is now as follows;
The protection system clearance time upstream of the substation will normally not clear in a short time (typically anywhere between 5 seconds to 5 minutes, dependant on the system). The inherent personal protection of the system therefore relies on the principle that the ‘touch’ voltage between frames will be negligible. That is, if you were to touch the 11kV switchgear frame and the fence or transformer frame (at the same time) only a small voltage potential would exist between your two hands. It is also my understanding that if you were to only touch the 11kV switchgear frame (or fence or transformer frame), then there would also be only (relatively) a small voltage potential between your hand(s) and your feet(s). Since the ground potential rise in the earth grid will raise the voltage potential under your feet to a level approaching that of the fence or switchgear frame. It is also my understanding that if you were touching nothing at all the ‘step’ potential between your feet will be a small voltage figure since the ground potential rise (although not uniform) will not significantly vary between your feet due to the earth grid. [In theory – this depends on the effectiveness of the earth grid model, variations in soil resistivity values etc].
The inherent equipment protection would rely on the thermal effects of the fault connection being less than what mechanically could be withstood by any of the equipment.
The first question is – is this ok - does everyone concur so far?
My main question is as follows - what happens to a ‘low voltage’ earth system external to the substation (at the MCC for instance), for the following situations;
1)no low impedance connection between the ‘low voltage’ earth system (external to the substation) and the earth system of the substation,
2)the situation where cable tray or cable ducts from the transformer (for the LV distribution cable) are bonded (purposely or otherwise) to the substation earth system and are then bonded (purposely or otherwise) to the ‘low voltage’ earth system (external to the substation),
My understanding of the situation would then be as follows;
For 1) ‘Transfer’ of voltages between the systems is unlikely to occur. That is, should there be a fault as described above, the 11kV conductor (6350V relative to ‘true’ earth under normal conditions) is not likely to transfer its potential to a remote system away from the substation.
For 2) ‘Transfer’ of voltages is likely. That is, potential rise of the earth system will occur on external equipment such as; MCC frame, MCC main earth, MCC earth grid, domestic earth connections for bonding between MCC and metallic objects such as water pipes, sinks etc.
Now again in this situation, for ‘touch’ voltages between frames, that is, hand-hand contact between two frames that are directly bonded, the potential is small (they are approximately at the same potential, bonding resistance being small). However, the ‘touch’ voltage for the situation where someone is touching an ‘energised’ earth frame and has their feet in contact with the ground (hand-foot path), may now be high. Since the ground under the person’s foot in a ‘domestic’ type situation in an industrial plant will not normally be well connected to the main earth system (no thorough earth grid under foot). [The basic theory behind ‘domestic’ earthing (as I understand it) is to prevent ‘touch’ potentials between conducting objects during a fault on the 415V system, and also for ‘improvement’ of the earth system resistance to earth. If the 415V faults can be cleared quick enough (by the main fault current protection equipment looking at the main fault currents) to prevent leakage currents hand-foot causing physiological problems, then this is deemed ok (the long term effect of circulating ground currents on domestic piping not withstanding). For this discussion, however, the fault cannot be cleared by the 415V board].
This situation practically may arise in ‘unit sub’ arrangements where transformers are physically incorporated into LV MCC panels and the MCC panel has bonded ‘domestic’ type earth connections, and a fault occurs between the 11kV connections of the transformer and the transformer frame, and hence to all other bonded connections.
Would you consider point 2) to be unsafe? Would you consider installations of type 2) common or uncommon? Additionally what is the effect on the neutral (and LV phases) in all these cases?
