Acceptable reading for a new grounding system

[rmr660]

In a general industrial application, what kind of reading is acceptable for a new grounding system?


5 ohms

10 ohms

15 ohms

25 ohms

I thought the correct answer was no greater that .5 Ohms

[Kalbi_Rob]

In a general industrial application, what kind of reading is acceptable for a new grounding system?


5 ohms

10 ohms

15 ohms

25 ohms

I thought the correct answer was no greater that .5 Ohms

2008 NEC:
250.56 Resistance of Rod, Pipe, and Plate Electrodes
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.

IEEE 142 (Green Book) 3.3.4.6:
For satisfactory lightning protection, substation grounding network resistance must
not exceed 5 ohm; for large stations, lower values are desirable.

IEEE 142 (Green Book) 4.1.3:
Resistances in the 1 ohm to 5 ohm range are generally found suitable for industrial plant substations and buildings and large commercial installations. Special grounding considerations for Arctic conditions will not be addressed, but additional information is available in Gill, and Beck and Yu.

The 25 ohm value noted in the NEC applies to the maximum resistance for a single
electrode consisting of a rod, pipe, or plate. If a higher resistance is obtained for a single
electrode, a second electrode of any of the types specified in the NEC is required. This
should not be interpreted to mean that 25 ohm is a satisfactory resistance value for a
grounding system.

So, for Industrial and Large Commercial Installations, ground resistance shall be less than 5 ohms, all other times less than 25 ohms. Any specification can call for lower resistances, but not more. Thus many Utilities will call for Substations to be less than 1 ohm or even 0.5 ohms.

[Kalbi_Rob]

2008 NEC:
250.56 Resistance of Rod, Pipe, and Plate Electrodes
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.

IEEE 142 (Green Book) 3.3.4.6:
For satisfactory lightning protection, substation grounding network resistance must
not exceed 5 ohm; for large stations, lower values are desirable.

IEEE 142 (Green Book) 4.1.3:
Resistances in the 1 ohm to 5 ohm range are generally found suitable for industrial plant substations and buildings and large commercial installations. Special grounding considerations for Arctic conditions will not be addressed, but additional information is available in Gill, and Beck and Yu.

The 25 ohm value noted in the NEC applies to the maximum resistance for a single
electrode consisting of a rod, pipe, or plate. If a higher resistance is obtained for a single
electrode, a second electrode of any of the types specified in the NEC is required. This
should not be interpreted to mean that 25 ohm is a satisfactory resistance value for a
grounding system.

So, for Industrial and Large Commercial Installations, ground resistance shall be less than 5 ohms, all other times less than 25 ohms. Any specification can call for lower resistances, but not more. Thus many Utilities will call for Substations to be less than 1 ohm or even 0.5 ohms.

ANSI/BICSI 002: Data Center Design and Implementation Best Practices

Recommends 5 Ohms Maximum, but recommends 3 Ohms for Class F2 & F3 Data Centers, and 1 Ohm for Class F4 Data Centers