2.Target structure for grounding

-Houses and structures with metallic roofs insulated to the ground. except( metal structures beneath the roofs)

-Houses and structures with metallic roof (including all metal parts such as walls, girders, beams, rafters and doors insulated to the ground)

-Metallic structures insulated to ground and installed outside houses and structures (e.g. iron frames, iron pipes TV antennas, clotheslines)

3.Requirement for grounding system below overhead power line

(1) Grounding rod

Grounding rod shall be round steel with diameter not less than 16mm (equivalent square steels) or angle steel with the size not smaller than 40mm*40mm*4mm. Grounding rod shall have the length not less than 1m and be piled so that the distance from the ground to the head of them is from 0.1m to 0.15m. Also, grounding rods are prohibited to be painted on the surface, and connection parts

between them and grounding conductors shall be made of coppers or be galvanized at corrosive areas.

(2) Grounding conductor

Grounding conductor shall be usedwith flexible copper conductors with cross-sectional area not less than 16mm2, steel rodswith diameter not less than 6mmor steel plates not smaller than24mm*4mm. Galvanized or stainless steel conductors are permitted to be used as grounding conductors. Also, grounding conductor shall be connected to grounding rods by welding, bolting or pressure bonding.

Chapter 6-3 Components to be Earthed in Electrical Equipment

Article 557. Components to be Earthed

(1) Metal base or enclosure of electrical equipment connected to the power gird shall be grounded. And, even if power transformer or instrument transformer doesn’t have enclosure, its iron core shall

be grounded. However, when such electrical equipment is surrounded by fence to prevent person from contacting such electrical equipment or is installed on an insulated base, the above-mentioned requirement cannot be applicable.

Usually, live part of electrical equipment is insulated from its metal base and metal enclosure. However, since short-circuit between the live part and the metal base or the metal possibly happens,

equipments shall be surrounded byprotective fence and installed on insulated base instead of grounding.

(2) Neutral point of the power gird can be grounded in order to ensure necessary performance of protective relays and restrain potential rise caused by earth fault.As for power grid with voltage

less than 300V or equal, if its neutral point is difficult to be grounded, any one phase conductor can be grounded instead of grounding neutral point.

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Grounding method of the neutral point can be selected among direct grounding and grounding through reactor or resistor from the viewpoint of ensuring detection of earth fault and disconnection

of fault part.On the other hand, grounding through an instrument transformer which has large impedance is not included in the grounding method of neutral point.

(3) Transformer which connects high-voltage power gird or middle-voltage power grid to low-voltage power grid shall be grounded in the following way.

-Grounding at the neutral point of the low-voltage side

-Grounding at any one phase conductor (if the neutral point is difficult to be grounded in the transformer with low-voltage side of less than 300V)

-Grounding at metal plate between high-voltage side and low-voltage side (if low-voltage side

grid is isolated)

In transformer which connects low-voltage grid to high-voltage grid, the potential in the lowvoltage grid may increase excessively in case that short-circuit between low-voltage grid and highvoltage gird happens. To reduce the potential rise in the low-voltage gird in such case, low-voltage grid shall be grounded.

Generally, grounding of the low-voltage grid should be done at the neutral pointHowever,. if the neutral point is difficult to be grounded due to delta connection or -phasesingle transformer, grounding of the low-voltage grid can be done at any one phase conductor.The reason is that, in such case, current unbalance in case of earth fault does not need to be considered.

On the other hand, grounding at any one phase conductor of transformer with low-voltage side of more than 300V causes considerable current unbalance in case of earth faultTherefore,. in such transformer, the neutral point of low-voltage grid shall be grounded.If low-voltage gird have to be connected in delta connection, such transformer shall have metal plate between high-voltage side and low-voltage side to isolate these two grids.

(4)The secondary circuit of high-voltage or middle-voltage instrument transformer shall be grounded. This purpose is that potential rise on the secondary circuit has to be restrained thwin permissible value in case of short-circuit between the primary circuit and the secondary circuit.

(5)If performance of grounding or protective disconnectionaccording to the regulation is difficult to be implemented, electrical equipment with insulation platform is allowed to be used.

The ungrounded parts of the insulation platform must not contact its grounded partsAdditionally,.

the ungrounded parts must not contact grounded parts of other equipment or structures of other buildings.

(6)Surrounding fence of electrical equipment must not be grounded. However, if an overhead line of 110kV or more is connected to the equipment, thesurrounding fence must be grounded by

between the fence and grounding device arranged along the fence shall not be less than 2 m. Grounding electrodes, metal sheath cables, metal pipes extending out of the area surrounded by the

fence shall be arranged at the middle point of the piers of the fence at the depth of 0.5m or more.

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Article 558. Protection of Transformers with primary coil voltage above 1kV and secondary coils to 1kV

Refer to item (3) of Guideline 557.

Chapter 6-4 Components Exempt from Earthing

Article 559. Components Exempt from Earthing

In case that earth fault doesn’t cause danger to people, grounding work can be omitted. Such cases are

as follows.

a) Electrical equipment with voltage of less than 380V or equal is installed on an insulated base.

-An insulated base is dry wooden floor, floor covered withlinoleum, stone, etc. Concrete floor is not included.

-However, in wet condition, this stipulation cannot be applied.

b)A metal enclosure of machine is connected to a grounded metal base electrically, and accordingly, its ground resistance is less than required value.

c)Metal enclosures of cable pipes or cable troughsearconnected electrically to each other, and accordingly, its ground resistance is less than required value.

d)A rail of an electrical railway is used for a part of electrical circuit.

-In order to prevent leakage current from causing electrical corrosion toetalm structures around the rail, the rail shall not be grounded.

-The ground resistance of the rail shall be increased by insert gravel or wooden plate between the rail and ground.

e)Portable electrical tool with a double insulation

Chapter 6-5 Protection against Earth Faults

Article 560. Protective Breaking Time on Effective Earthed Neutral

The power grid shall be equipped with a protective relay, which interrupts the fault part in case of earth fault on the power grid in order to avoid danger caused by the earth fault and interruption in electric power supply.

a)The earth fault protective relay shall be equipped on the power line used to supply power to the machine with voltage of more than 60V, which has a metal enclosureThe. earth fault protective

relay shall interrupt fault part automatically in case of earth fault. However, this requirement is not applied to lighting circuit and control circuit.

b)When power line is connected to low-voltage side of an insulation transformer, of which voltage is not more than 300V, and this power line is not grounded, it is not necessarily equippedwith the earth fault protective relay.

c)When power line with voltage of more than 300V is connected to middle-voltage or high-voltage

d)Middle-voltage or high-voltage power line shall be equipped with the earth fault protective relay according to the following table.

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e)When middle-voltage or low-voltage power line supply power to the equipment used for public security such as traffic signal, emergency lighting, etc., this power line is not necessarily equipped with the earth fault protective relay under the condition that alarm signal is sent to dispatching control center in case of earth fault.

The breaking time is summation of the two durations below.

-Operation time of the earth fault protective relay: From detection of fault to transmitting signal for interruption

-Operation time of circuit breaker: from receiving the signal to finishing interruption

If the following requirements on protection against earth fault are satisfied, isolated earthed points are allowed to be used for electrical power line with voltage up to 1kV.

a.Power line shall be equipped with protective devices in combination with insulation tester of the power line. Circuit breaker or fuse can be used for protection.

b.It is possible to detect earth fault quickly and extinguish it timely. Automatic circuit breaker which interrupts the fault part shall be equipped.

Article 561. Detection of Earth Faults in Transformers with Isolated Neutral

Power line with isolated neutral point shall be equipped with earth fault protective relay in accordance with Article 564.

Main protective relay shall be a ground directional relay in order to enable selective trip for earth fault. Since operating sensitivity of the ground directional relay is maybe insufficient for earth fault with large ground resistance at the fault point, ground overvoltage relay shall be equipped on the power line as a back-up relay.

For isolated neutral electrical equipment,the setting value of one-phase ground protective relay shall be determined according to calculated earth-fault current.

At the determination of setting value of the protective relay or use,f the calculated earth-fault current shall not be less than 1.5 times of the setting value of protection relay or 3 times of the rated current of fuse wire.

Article 562. Detection of Earth Faults in Transformers with Earthed Neutral

Power line with effective grounded neutral shall be equipped with earth fault protective relay in accordance with Article 564.

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