Interview Question on Electrical Grounding

What is grounding?

It is an electrical connection with the general mass of earth through an earth electrode.

What is difference between earthing and grounding?

Both have same meaning. The term earthing is used in U.K. and grounding in U.S.A. ground means earth.

What are types of grounding?

There are two types

1. System grounding.
2. Equipment grounding.

What does mean by system?

Grounding of neutral point of equipment is called system grounding. For instance grounding of generator neutral, transformer neutral etc.

What does mean by equipment grounding?

Grounding of non-current carrying metallic parts is called equipment grounding. For instance no-current carrying parts include the following:

1. Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc.
2. Support structures, tower, poles etc. in the neighborhood of electrical circuits.
3. Sheath of cables.
4. Body of portable equipment such as iron, oven, etc.

What is the important of system grounding?

It is important because:

1. Earth fault protection is based on the method of neutral earthing.
2. System voltage during earth fault depends on neutral earthing.
3. It is a protection against arcing grounds, unbalanced voltages with respect to earth and lighting.

What is the important of equipment grounding?

Equipment earthing ensures safety.

How safety could be ensured by equipment grounding?

In order to enumerate this, let us first find out the effects of current and voltage developed during fault condition.

What is the permissible body current limit?

The magnitude and duration of current conducted through a human body at 50Hzshould be less than those did that cause ventricular fibrillation.(Ventricular fibrillation is considered to be the main cause of death due to electrical shock).These below given data are also applicable for current limits to human body.

Why grounding is necessary?

The purpose of grounding is to maintain the surface under and around a station ate as nearly zero potential as possible with reference to absolute earth so that operating staff who walk in the station yard and tough equipments are ate earth potential and when faults occur there is safety to staff and equipments.

What are the harms of under grounded system?

1. Step and tough potential will increase more than maximum tolerable value.
2. Under single line to ground fault the voltage to earth of the two healthy phases rises from their normal phase to neutral voltage to full line voltage, which may result in insulation break down.
3. The capacitive current in two healthy phases increases ª3 times the normal value.
4. The capacitive current in the faulty phase is 3 times its normal value.
5. Experience shows that capacitive current in excess of 4 amps may be sufficient to maintain an arc in the ionized path of the fault and this persistent arc phenomenon is called ARCING GROUND, which ultimately cause high voltage build up. Some time these voltage builds up to 5 to 6 times its normal value, which results in break down of insulation.
6. Being fault current low, it is difficult to isolate fault.

How system grounding and equipment grounding achieved?

System grounding is obtained by grounding the neutral through resistance, through transformer and through effective or solidly grounding. Equipment grounding is obtained by Grounding of non-current carrying metallic parts equipment. For instance no-current carrying parts include the following:

1. Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc.
2. Support structures, tower, poles etc. in the neighborhood of electrical circuits.
3. Sheath of cables.
4. Body of portable equipment such as iron, oven, etc.

What does mean by grounding electrode, grounding system, and grounding resistance?

Grounding electrode: A conductor driven in the earth and used for collecting ground current from or dissipating ground current into the earth.

Grounding system: Comprises all interconnected grounding facilities in a specific area.

Grounding resistance: The resistance offered by the ground when power frequency current is discharged to the ground through a particular grounding electrode or grounding system.

How grounding resistance could be measured?

There are few methods, which can give approximately true value. These are

described below.

1. Fall of potential method: This method is applicable for small grid or sub station where induction effect of voltage is less.
2. Measurement of earth resistance by 61.8% distance rule:
3. Alternate – 1 of fall of potential method: This method is influenced by induction effect.
4. Alternate – 2 of fall of potential method:

How value of grounding resistance could be kept constant?

While measuring of grounding resistance is more than computed design value0.11Ω,then following are recommended to reduce it. Add in water the following highly conductive substances and pour into treated pit.

1. Sodium chloride (Nacl), known as common salt.
2. Calcium chloride (Ca CL2)
3. Sodium carbonate (Na2 CO3)
4. Copper sulphate (Cu SO4)
5. Soft choke and
6. Salt and charcoal in suitable proportions.

What is the effect of moisture content on earth resistivity?

The moisture content is expressed in percent by weight of dry soil. Dry earth weights about 1440 kg per cubic meter and thus 10% moisture content is equivalent to 144 kg of water per cubic meter of dry soil. So about 20% moisture, the resistivity is very little affected. Below 20%, the resistivity increases very abruptly with the decrease in moisture.

What is the effect of salt content in moisture on resistivity?

The resistivity decreases and the salt content is expressed in percent by weight of the contained moisture. It will be noted that the curve flattens off at about 5% salt content and a further increase in salt content gives little decrease in the soil resistivity.

What is the effect of temperature on earth resistivity?

The temperature co-efficient of resistivity for soil is negative, but it is negligible for temperature above freezing point. Below 0°C the water in the soil begins to freeze and introduces a tremendous increase in the temperature co-efficient, so that as the temperature becomes lower the resistivity rises enormously.

What does mean by neutral floating or neutral displacement?

When a ground fault occurs, there is a tendency of neutral shift with consequent change in voltage on the un-faulted phases. This phenomenon is called neutral floating or neutral displacement.

In 220 kV switchyard why lightning arrestor should be properly grounded?

1. During lightning, surges should be discharged to ground, otherwise it will puncture the equipment insulation and it is possible only when lightning arrestor is grounded properly.

3. In order to make it effective, the ground terminal of lighting arrestor should be connected direct to the tank of transformer. This will eliminate voltage build up due to earth resistance. For example for each ohm of earth resistance the voltage build up for 5000 Amps discharge current is 5 kV. Soil resistivity a should be minimum and may be it is 3.5 ohm per meter.

Why grounding mat is important near ground switch operating handle and disconnecting switch operating handle?

Equipment operating handles deserve special attention because of the higher probability for co-incidence of adverse factors. For example,

1. Hand operation equipment such as grounding switches and disconnecting switches requires the presence of operator near a grounded structure at a point where opening of an energised circuit can some times result in an arc to the structure or perhaps mechanical failure and electrical break down of a switch insulator. A large percentage of fatal accidents from voltage gradients are in fact associated with operating handles. Hence in order to avoid above problems following should be an additional safety factors:

1. Use closer mesh in the vicinity of operating handle area (150-mm approx.) and operating handle shall be directly connected to the earthing mat.
2. Use higher resistance surfacing such as crushed rock or both in order to bringdown the values of touch potential and step potential.

Why copper is only preferred as material for grounding?

An advantage of use of copper is in addition to their high conductivity, has the advantage of being resistant to underground corrosion. Copper is cathodic with respect to other metals that are likely to be burried in the vicinity.

Disadvantages of use of copper are,

1. Grid of copper forms a galvanic cell with burried steel structures, pipes and any of the lead based alloys that might be present in cable sheaths, it is likely be hasten the corrosion of the latter.

1. Use of tinned copper conductor accelerates and concentrates the natural corrosion of metal in small area however cell potential with respect to steel and zinc reduces by about 50% and practically eliminates this potential with respect to lead.

How much resistance human body has?

Resistance of internal body tissues (Not including skin) : 300 Ω.

Resistance of body including skin : 500 to 3000 Ω.

What is the effect of voltage frequency and current on resistance of the human body?

1. For touch voltages up to approximately 50V the value of impedance of the skin varies widely with surface area of contact, temperature, respiration etc. even for one person.
2. For higher touch voltages in order of approximately 50V to 100V the skin

impedance decreases considerably and becomes negligible when the skin breaks down.

1. Wet hand contact resistance becomes very low at any voltage.
2. With increase in frequency, impedance of skin decreases.

What are the paths of current through the body?

A value of 1000 Ωis selected for the calculations that follows as representing the resistance of a human body from hand to both feet and also from hand to hand or from one foot to other foot.

Above paths includes vital organs such as heart.

1. Path from hand to foot is much more dangerous than foot to foot, since current flow through heart during foot to foot current flow will be much less than the current flow from hand to foot approximate ratio is 25:1
2. However deaths have occurred during foot to foot current flow. Hence can not be ignored.

What maintenance of earth electrodes should be done?

The neighbouring soil to the earth electrode shall be kept moist where necessary, by periodically pouring water through a pipe where fitted along with it or by pouring water in the immediate vicinity of the earth electrode.

Periodical visual inspection of all earth electrodes connection wherever available, shall be carried out to ensure their rigidity and other signs of deterioration.

What are the effects of re-closure shock?

During re-closure, when fault is persisting a person might be subjected to the first shock which would not permanently injure him, but would upset and disturb him temporarily. Next, a single fast automatic re-closure could in a second shock initiated within less than 500 ms from the start of first. It is this second shock, occurring after a relatively short interval of time before the person has recovered, that might cause a serious accident. With manual re-closure the possibility of exposure to a second shock is reduced since the time interval may be substantially greater.

Why AC is more dangerous than DC?

Because the excitatory action of current (stimulation of nerves and muscle, induction of cardiac atrial or ventricular fibrillation) are linked to the changes of current magnitude especially when making and breaking of the current. To produce the same excitatory effects the magnitude of direct current flow of constant strength in 2 to 4times greater than that of alternating current.