Power Systems Protection and Relaying code numbers

In North America protective relays are generally referred to by standard device numbers. Letters

are sometimes added to specify the application (IEEE Standard C37.2-2008).

Following the ANSI/IEEE Standard Device Numbers (the more commonly used ones are in bold)

1 - Master Element
2 - Time Delay Starting or Closing Relay
3 - Checking or Interlocking Relay
4 - Master Contactor
5 - Stopping Device
6 - Starting Circuit Breaker
7 – Rate of Change Relay
8 - Control Power Disconnecting Device
9 - Reversing Device
10 - Unit Sequence Switch
11 – Multifunction Device
12 - Overspeed Device
13 - Synchronous-speed Device
14 - Underspeed Device
15 - Speed or Frequency-Matching Device
16 – Data Communications Device
20 - Elect. operated valve (solenoid valve)
21 - Distance Relay
23 - Temperature Control Device
24 – Volts per Hertz Relay
25 - Synchronizing or Synchronism-Check Device
26 - Apparatus Thermal Device
27 - Undervoltage Relay
30 - Annunciator Relay
32 - Directional Power Relay
36 - Polarity or Polarizing Voltage Devices
37 - Undercurrent or Underpower Relay
38 - Bearing Protective Device
39 - Mechanical Conduction Monitor
40 –Field (over/under excitation) Relay
41 - Field Circuit Breaker
42 - Running Circuit Breaker
43 - Manual Transfer or Selector Device
46 – Rev. phase or Phase-Bal. Current Relay
47 - Phase-Seq. or Phase-Bal. Voltage Relay
48 - Incomplete-Sequence Relay
49 - Machine or Transformer Thermal Relay
50 - Instantaneous Overcurrent
51 - AC Time Overcurrent Relay
52 - AC Circuit Breaker
53 – Field Excitation Relay
55 - Power Factor Relay
56 - Field Application Relay
59 - Overvoltage Relay
60 - Voltage or Current Balance Relay
62 - Time-Delay Stopping or Opening Relay
63 - Pressure Switch
64 - Ground Detector Relay
65 - Governor
66 – Notching or jogging device
67 - AC Directional Overcurrent Relay
68 - Blocking or “out of step” Relay
69 - Permissive Control Device
74 - Alarm Relay
75 - Position Changing Mechanism
76 - DC Overcurrent Relay
78 - Phase-Angle Measuring Relay
79 - AC-Reclosing Relay
81 - Frequency Relay
83 - Automatic Selective Control or Transfer Relay
84 - Operating Mechanism
85 – Pliot Communications, Carrier or Pilot-Wire Relay
86 - Lockout Relay
87 - Differential Protective Relay
89 - Line Switch
90 - Regulating Device
91 - Voltage Directional Relay
92 - Voltage and Power Directional Relay
94 - Tripping or Trip-Free Relay
B – Bus
F - Field
G – Ground or generator
N – Neutral
T – Transformer

Standard diesel engine cycle

The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the fuel to ignite the fuel rather than using a spark plug ("compression ignition" rather than "spark ignition").

Air standard diesel engine cycle

In the diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression raises the temperature to the ignition temperature of the fuel mixture which is formed by injecting fuel once the air is compressed. The ideal air-standard cycle is modeled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and an iso volumetric exhaust. A new air charge is taken in at the end of the exhaust, as indicated by the processes a-e-a on the diagram.

Since the compression and power strokes of this idealized cycle are adiabatic, the efficiency can be calculated from the constant pressure and constant volume processes. The input and output energies and the efficiency can be calculated from the temperatures and specific heats:

It is convenient to express this efficiency in terms of the compression ratio r_C = V₁/V₂ and the expansion ratio r_E = V₁/V₃. The efficiency can be written
and this can be rearranged to the form

The difference between TBN and TAN

TBN is total base number and TAN is total acid number. TBN is a measure of the reserve alkalinity or reserve acid neutralization remaining in the oil. TAN measure the increase of oil oxidation and build-up of corrosive acidic compounds. Engine manufacturers often recommend utilizing both tests to gain a more in depth understanding of oil condition and engine oil remaining protection. In utilizing both tests, the TBN will decrease over time and TAN will increase over time. The point where the two numbers meet or cross over can be considered the point where the oil can no longer provided adequate corrosive where protection.

TBN In Diesel Engine Oils

TBN is an important property of engine oils. The abstract definition is as follows;

“Total Base Number (TBN) is the quantity of acid, expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to neutralize all basic constituents present in 1 gram of sample (ASTM Designation D 974)”. But this tells us little about what TBN in an engine oil does, nor how much we need for effective engine oil performance and engine protection.

The detergent additive in an engine oil has two functions

• To control deposits in the hot parts of the engine such as the pistons and turbocharger bearings.

• To neutralize acidic products of combustion from the fuel that can cause corrosive wear.

Engine oil formulators have always matched the amount of TBN to the amount of sulfur in the fuel. Today Chevron manufactures engine oils with 70 TBN which are used in marine engines operating on 5% sulfur fuel. This is very high sulfur content, 50,000 parts per million. Diesel fuel in the US was approximately 2,500 to 3,000 ppm sulfur (the legal maximum for ASTM 2D fuel was 5000 ppm) until 1993, when EPA regulations required a reduction to a maximum limit of 500 ppm for on road use. Today all diesel fuel is limited to 15 ppm sulfur maximum (Ultra Low Sulfur Diesel, or ULSD).

With 3000 ppm sulfur diesel fuel, oil TBN in the range of 10 to 14 was common, with lower priced oils at approximately 8 TBN. Current engine oils for use with ULSD are around 8 to 9+. Clearly the need for high TBN does not exist with today’s ULSD fuels.

How is TBN measured? It is important to note there are several test methods for Total Base Number. The one used in product data sheets is generally ASTM D 2896. This method uses perchloric acid to neutralize the alkalinity in the oil and yields a slightly higher number than the test method used by the oil analysis labs. They generally use ASTM D 4739 and the acid used here is hydrochloric acid. This produces a number approximately 2 mg KOH/g LOWER than ASTM D 2896 for the same oil. Due to chemical interferences, this test method does not recognize all of the alkalinity that ASTM D 2796 sees.

Why are there two test methods? The oil manufacturers have typically used ASTM D 2896 and their labs are set up to handle perchloric acid, which is toxic and hard to handle. In addition ASTM D 2896 can measure both the “hard base” from metallic detergent as well as the “soft base” from organic, non- metallic ingredients. So it is a more accurate method. BUT, the production oil analysis labs prefer to use a safer and easier to use titration acid, namely hydrochloric acid. The tests can be run faster, more cost effectively and more safely.

How much TBN do we need to protect the engine? The old rule was to change the engine oil when 50% of the new oil TBN had been consumed. Because of the virtual absence of fuel sulfur today, much less is needed. Chevron now sets the TBN guidelines for all of its diesel engine oils as follows:

FOR ALL OILS when using ULSD

• Severity 1: 50%-44% of new oil TBN or 3.5 to 4

• Severity 2: 43%-36% of new oil TBN 3 48 to 2.9

• Severity 3: <35% of new oil TBN < 2.8 to 2

• Severity 4: less than 2 <2

Other parameters of engine oil are now more important to engine durability and extended service protection than TBN. These are parameters such as oxidation stability, wear control, effective soot dispersancy. A balanced oil has multiple performance abilities and TBN is only one of the performance measures that are important in today’s high performance engine oil.

Power Plant Generators: What is Excitation?

Electric generators work on the principle of Faraday’s electromagnetic induction. The essential part of this principle is the magnetic field. The magnetic filed is produced from a DC power source from an Exciter that is part of the generator system.

The main requirement for electricity generation as per the basic principle is a magnetic field. The generator while producing electricity also has to produce this at a constant voltage for the electrical system to work properly. Controlling the magnetic field controls the voltage output of the generator.

• How does one produce and control this magnetic field in a large generator?
• The rotor or the field coils in a generator produce the magnetic flux that is essential to the production of the electric power. The rotor is a rotating electromagnet that requires a DC ( Direct Current) electric power source to excite the magnetic field. This power comes from an exciter.

• DC Exciter

  In the past, the exciter was a small DC generator coupled to the same shaft as the rotor. Therefore, when the rotor rotates this exciter produces the power for the electromagnet. Control of the exciter output is done by varying the field current of the exciter. This output from the exiter then controls the magnetic field of the rotor to produce a constant voltage output by the generator. This DC current feeds to the rotor through slip rings.

• Static Exciter

  In modern generators the exciters are static. The DC power for the electromagnet is from the main generator output itself. A number of high power thyristors rectify the AC current to produce a DC current which feeds to the rotor through slip rings. This eliminates the operation and maintenance problems associated with having another rotating machine. Static exciters offer a better control of the output than an electromechanical control.
• • During start up, when there is no output from the generator, a large battery bank provides the necessary power for excitation.

  • Brushless Exciter

    Another method is the brushless system. In this system the armature of the exciter is on the rotor shaft itself. The DC output of this armature, after rectification by solid-state devices, goes to the rotor coils. Since the armature and rotor are on the same rotating shaft, this eliminates the need for slip rings. Hence it reduces maintenance and operational requirements and thus improving reliability.
  • 
    
• 
  

Class of insulation

The main component for electric motor is a stator. What is stator? Basically stators are wound with insulated windings made from cooper wire. The insulation materials for winding of stator such as polyester, poly vinyl formal, polyurethane etc.

The main purpose of insulation is to protect the windings in the slots of the stator lamination and layer between winding coils. The insulation class is durability factor depend on whole of insulation condition.

According from IEEE regulation, the classification of insulation electric motor have a deference rating for maximum temperature that insulation winding can operate. We can see the insulation class at  motor nameplate. Please refer the table below for insulation class rating temperature.

Classification of insulation for electric motor:

Electric motor manufactures design the winding insulation class depend on technical aspect and user application. For good grade of insulation, the cost is higher. So the engineer selects the electric motor based on applications and working area temperature characteristics.

The temperature for electric motor is depending on the fans performance and design. Manufacture normally design the cooling fan according to the duty, type of application and load on the motor. Theoretically the load on motor affected the temperature of motor.

The conclusion is, when we selected the class of insulation for electric motor, several factor must be consider are:

§  Type of load ( pump, turbine, compressor)

§  Working area ambient temperature

§  Application of motor

§  Room condition

How can control power factor in power plant ?

When the synchronous generator is pushing reactive power into the electrical system we say the machine is over-excited. When the synchronous generator is absorbing reactive power from the electrical system, it is under-excited. So, the reactive power output of the generator is associated with the generator field current, provided by the excitation system. 

But note that we do not control reactive power output of a generator, at least in terms of automatic control. Synchronous machines are usually operated on automatic voltage regulator (AVR) mode, so we are actually controlling voltage. The reactive power output of the generator is thus automatically adjusted as needed to maintain the specified voltage.

Conversely, if you try to control reactive power output or power factor in a generator, you will not be able to control voltage.

And we should operate the synchronous generators on AVR because of stability, the AVR is a proven solution to several stability issues (steady state stability and transient stability).

Advantages of Electrical Drives

Electrical drives are readily used these days for controlling purpose but this is not the only the advantage of Electrical drives. There are several other advantages which are listed below – 

1) These drives are available in wide range torque, speed and power.
2) The control characteristics of these drives are flexible. According to load requirements these can be shaped to steady state and dynamic characteristics. As well as speed control, electric braking, gearing, starting many things can be accomplished.
3) The are adaptable to any type of operating conditions, no matter how much vigorous or rough it is.
4) They can operate in all the four quadrants of speed torque plane, which is not applicable for other prime movers.
5) They do not pollute the environment.
6) They do not need refueling or preheating, they can be started instantly and can be loaded immediately.
7) They are powered by electrical energy which is atmosphere friendly and cheap source of power.

Parts of Electrical Drives

The diagram which shows the basic circuit design and components of a drive, also shows that, drives have some fixed parts such as, load, motor, Power modulator control unit and source. These equipments are termed as parts of drive syatem. . Now, loads can be of various types i.e they can have specific requirements and multiple conditions, which are discussed later, first of all we will discuss about the other four parts of electrical drives i.e motor, power modulators, sources and control units.

Electric motors are of various types. The dc motors can be divided in four types – shunt wound dc motor, series wound dc motor, compound wound dc motor and permanent magnet dc motor. And AC motors are of two types – induction motors and synchronous motors. Now synchronous motors are of two types – round field and permanent magnet. Induction motors are also of two types – squirrel cage and wound motor. Besides all of these, stepper motors and switched reluctance motors are also considered as the parts of drive system.

So, there are various types of electric motors, and they are used according to their specifications and uses. When the electrical drives were not so popular, induction and synchronous motors were usually implemented only where fixed or constant speed was the only requirement. And for variable speed drive applications, dc motors were used. But as we know that, induction motors of same rating as a dc motors have various advantages like they have lighter weight, lower cost, lower volume and there is less restriction on maximum voltage, speed and power ratings. For these reasons, the induction motors are rapidly replaced the dc motors. Moreover induction motors are mechanical stronger and requires less maintenance. When synchronous motors are considered, wound field and permanent magnet synchronous motors have higher full load efficiency and power factor than induction motors, but the size and cost of synchronous motors are higher than induction motors for the same rating.

Brush less dc motors are similar to permanent magnet synchronous motors. They are used for servo applications and now a days used as an effcient alternative to dc servo motors because they don’t have the disadvantages like commentation problem. Beside of these, stepper motors are used for position control and switched reluctance motors are used for speed control.

Power Modulators - are the devices which alter the nature or frequency as well as changes the intensity of power to control electrical drives. Roughly, power modulators can be classified into three types,
i) Converters,
ii) Variable impedance,
iii) Switching circuits.

As the name suggests, converters are used to convert currents from one type to other type. Depending on the type of function, converters can be divided into 5 types –

i. AC to DC converters

ii. AC regulators

iii. Choppers or DC-DC converters

iv. Inverters

v. Cycloconverters

AC to DC converters are used to obtain fixed dc supply from the ac supply of fixed voltage. The very basic diagram of ac to dc converters is like.

AC to DC Converter

Ac Regulators are used to obtain the regulated ac voltage, mainly auto transformers or tap changer transformers are used in this regulators. 

AC to AC Converter

Choppers or dc-dc converters are used to get a variable DC voltage. Power transistors, IGBT’s, GPO’s, power MOSFET’s are mainly used for this purpose. 

DC to DC Converter

Inverters are used to get ac from dc, the operation is just opposite to that of ac to dc converters. PWM semiconductors are used to invert the current. 

DC to AC Converter

Cycloconverters are used to convert the fixed frequency and fixed voltage ac into variable frequency and variable voltage ac. Thyristors are used in these converters to control the firing signals.

AC to AC Converter

Variable Impedances are used to controlling speed by varying the resistance or impedance of the circuit. But these controlling methods are used in low cost dc and ac drives. There can be two or more steps which can be controlled manually or automatically with the help of contactors. To limit the starting current inductors are used in ac motors.

Switching circuits in motors and electrical drives are used for running the motor smoothly and they also protects the machine during faults. These circuits are used for changing the quadrant of operations during the running condition of a motor. And these circuits are implemented to operate the motor and drives according to predetermined sequence, to provide interlocking, to disconnect the motor from the main circuit during any abnormal condition or faults.

Sources may be of 1 phase and 3 phase. 50 Hz ac supply is the most common type of electricity supplied in India, both for domestic and commercial purpose. Synchronous motors which are fed 50 Hz supply have maximum speed up to 3000 rpm, and for getting higher speeds higher frequency supply is needed. Motors of low and medium powers are fed from 400V supply, and higher ratings like 3.3 kv, 6.6 kv, 11 kv etc are provided also.

Control unit – choice of control unit depends upon the type of power modulator that is used. These are of many types, like when semiconductor converters are used, then the control unit consists of firing circuits, which employ linear devices and microprocessors.
So, the above discussion provides us a simple concept about the several parts of electrical drive.

Classification of Electrical Drives

The classification of electrical drives can be done depending upon the various components of the drive system. Now according to the design, the drives can be classified into three types such as single-motor drive, group motor drive and multi motor drive. The single motor types are the very basic type of drive which are mainly used in simple metal working, house hold appliances etc. Group electric drives are used in modern industries because of various complexities. Multi motor drives are used in heavy industries or where multiple motoring units are required such as railway transport. If we divide from another point of view, these drives are of two types:
i) Reversible types and
ii) Non reversible types.
This depends mainly on the capability of the drive system to alter the direction of the flux generated. So, several classification of drive is discussed above.

Safety Clearance for Transformer

Clearance from Outdoor Liquid Insulated Transformers to Buildings (NEC):

Liquid	Liquid Volume (m3)	Fire Resistant Wall	Non-Combustible Wall	Combustible Wall	Vertical Distance
Less Flammable	NA	0.9 Meter	0.9 Meter	0.9 Meter	0.9 Meter
	<38 m3	1.5 Meter	1.5 Meter	7.6 Meter	7.6 Meter
	>38 m3	4.6 Meter	4.6 Meter	15.2 Meter	15.2 Meter
Mineral Oil	<1.9 m3	1.5 Meter	4.6 Meter	7.6 Meter	7.6 Meter
	1.9 m3 to 19 m3	4.6 Meter	7.6 Meter	15.2 Meter	15.2 Meter
	> 19 m3	7.6 Meter	15.2 Meter	30.5 Meter	30.5 Meter

 Clearance between Two Outdoor Liquid Insulated Transformers (NEC):

Liquid	Liquid Volume (m3)	Distance
Less Flammable	NA	0.9 Meter
	<38 m3	1.5 Meter
	>38 m3	7.6 Meter
Mineral Oil	<1.9 m3	1.5 Meter
	1.9 m3 to 19 m3	7.6 Meter
	> 19 m3	15.2 Meter

 Dry Type Transformer in Indoor Installation (NES 420.21):

Voltage	Distance  (min)
Up to 112.5 KVA	300 mm (12 in.) from combustible material unless separated from the combustible material by a heat-insulated barrier.
Above 112.5 KVA	Installed in a transformer room of fire-resistant construction.
Above 112.5 KVA with Class 155 Insulation	separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Dry Type Transformer in Outdoor Installation (NES 420.22):

Voltage	Distance  (min)
Above 112.5 KVA with Class 155 Insulation	separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Non Flammable Liquid-Insulated Transformer in Indoor Installation (NES 420.21):

Voltage	Distance  (min)
Over 35KV	Installed indoors Vault (Having liquid confinement area and a pressure-relief vent for absorbing any gases generated by arcing inside the tank, the pressure-relief vent shall be connected to a chimney or flue that will carry such gases to an environmentally safe area
Above 112.5 KVA	Installed in a transformer room of fire-resistant construction.
Above 112.5 KVA (Class 155 Insulation)	separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Oil Insulated Transformer in Indoor Installation (NES 420.25):

Voltage	Distance  (min)
Up to 112.5 KVA	Installed indoors Vault (With construction of reinforced concrete that is not less than 100 mm (4 in.) thick.
Up to 10 KVA & Up to 600V	Vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting
Up to 75 KVA & Up to 600V	Vault shall not be required if where the surroundingStructure is classified as fire-resistant construction.
Furnace transformers (Up to 75 kVA)	Installed without a vault in a building or room of fire resistant construction

 Transformer Clearance from Building (IEEE Stand):

Transformer	Distance from Building  (min)
Up to 75 KVA	3.0 Meter
75 KVA to 333 KVA	6.0 Meter
More than 333 KVA	9.0 Meter

 Transformer Clearance Specifications (Stand: Georgia Power Company):

Description of Clearance	Distance  (min)
Clearance in front of the transformer	3.0 Meter
Between Two pad mounted transformers (including Cooling fin)	2.1  Meter
Between Transformer and Trees, shrubs, vegetation( for unrestricted natural cooling )	3.0 Meter
The edge of the concrete transformer pad to nearest the building	4.2 Meter
The edge of the concrete transformer pad to nearest  building wall, windows, or other openings	3.0 Meter
Clearance from the transformer to edge of (or Canopy) building (3 or less stories)	3.0 Meter
Clearance in front of the transformer doors and on the left side of the transformer, looking at it from the front. (For operation of protective and switching devices on the unit.)	3.0 Meter
Gas service meter relief vents.	0.9 Meter
Fire sprinkler values, standpipes and fire hydrants	1.8 Meter
The water’s edge of a swimming pool or any body of water.	4.5 Meter
Facilities used to dispense hazardous liquids or gases	6.0 Meter
Facilities used to store hazardous liquids or gases	3.0 Meter
Clear vehicle passageway at all times, immediately adjacent of Transformer	3.6 Meter
Fire safety clearances can be reduced by building a suitable masonry fire barrier wall (2.7 Meter wide and 4.5 Meter Tall) 0.9 Meter from the back or side of the Pad Mounted Transformer  to the side of the combustible wall
Front of the transformer must face away from the building.

Clearance of Transformer-Cable-Overhead Line (Stand: Georgia Power Company):

Description of Clearance	Horizontal Distance (mm)
	to pad-mounted transformers	to buried HV cable	to overhead HV Line
Fuel tanks	7.5 Meter	1.5 Meter	7.5 Meter
Granaries	6.0 Meter	0.6 Meter	15 Meter
Homes	6.0 Meter	0.6 Meter	15 Meter
Barns, sheds, garages	6.0 Meter	0.6 Meter	15 Meter
Water wells	1.5 Meter	1.5 Meter	15 Meter
Antennas	3.0 Meter	0.6 Meter	Height of Antenna + 3.0 Meter

Safety Clearance for Electrical Panel

 Working Space around Indoor Panel/Circuit Board (NES 312.2):

Voltage	Exposed live parts to Not live parts( or grounded parts )	Exposed live parts to Grounded parts (concrete, brick, and walls).	Exposed live parts on both sides
Up to 150 V	0.914 Meter (3 Ft)	0.914 Meter (3 Ft)	0.914 Meter (3 Ft)
150 V to 600 V	0.914 Meter (3 Ft)	1.07 Meter (3’6”)	1.22 Meter (4 Ft)

 Clearance around an Indoor electrical panel (NES 110.26):

Description of Clearance	Distance  (min)
Left to Right the minimum clearance	0.9 Meter (3 Ft)
Distance between Panel and wall	1.0 Meter
Distance between Panel and Ceiling	0.9 Meter
Clear Height in front of Panel>480V	2.0 Meter
Clear Height  in front of Panel <480V	0.9 Meter (3 Ft)
Clearance When Facing Other Electrical Panels < 480V	0.9 Meter (3 Ft)
The width of the workingspace in front of the Panel	The width of Panel or 0.762 Meter which is Greater.
Headroom of working spaces for panel boards (Up to 200Amp)	Up to 2 Meter
Headroom of working spaces for panel boards (More than 200Amp &Panel height is max 2 Meter)	Up to 2 Meter( If Panel height is max 2 Meter)
Headroom of working spaces for panel boards (More than 200Amp &Panel height is more than 2 Meter)	If Panel height is more than 2 Meter than clearance should not less than panel Height
Entrance For Panel (More than 1200 Amp and over 1.8 m Wide)	One entrance required for working space (Not less than 610 mm wide and 2.0 m high )
Personal Door For Panel (More than 1200 Amp)	Personnel door(s) intended for entrance to and egress from the working space less than 7.6 m from the nearest edge of the working space
Dedicated Electrical Space.	Required Space is width and depth of the Panel and extending from the floor to a height of 1.8 m (6 ft) above the equipment or to the structural ceiling, whichever is lower
The door(s) shall open in the direction of egress and be equipped with panic bars, pressure plates, or other devices that are normally latched but open under simple pressure
the work space shall permit at least a 90 degree opening of equipment doors or hinged panels

 Clearance for Conductor Entering in Panel (NES 408.5):

Description of Clearance	Distance  (min)
Spacing between The conduit or raceways(including their end fittings) and Bottom of Enclosure	Not rise more than 75 mm (3 in) above the bottom of the enclosure
Spacing Between Bottom of Enclosure and Insulated bus bars, their supports,	200 mm
Spacing Between Bottom of Enclosure and Non insulated bus bars	200 mm

 Clearance between Bare Metal Bus bar in Panel (NES 408.5):

Voltage	Opposite PolarityMounted on Same Surface	Opposite PolarityWhere Held Free in Air	Live Parts to Ground
Up to 125 V	19.1 mm	12.7 mm	12.7 mm
125 V to 250 V	31.8 mm	19.1 mm	12.7 mm
250 V to 600 V	50.8 mm	25.4 mm	25.4 mm

 Clearance of Outdoor electrical panel to Fence/Wall (NES 110.31):

Voltage	Distance  (min)
600 V to 13.8 KV	3.05 Meter
13.8 K V to 230  KV	4.57 Meter
Above 230 KV	5.49 Meter

 Working Space around Indoor Panel/Circuit Board (NES 110.34):

Voltage	Exposed live parts to Not live parts( or grounded parts )	Exposed live parts to Grounded parts (concrete, brick, and walls).	Exposed live parts on both sides
601 V to 2.5 K V	0.914 Meter (3 Ft)	1.2 Meter (4 Ft)	1.5 Meter (5 Ft)
2.5 K V to 9.0 K V	1.2 Meter (4 Ft)	1.5 Meter (5 Ft)	1.8 Meter (6 Ft)
9.0 K V to 25 K V	1.5 Meter (5 Ft)	1.8 Meter (6 Ft)	2.5 Meter (8 Ft)
25 K V to 75 K V	1.8 Meter (6 Ft)	2.5 Meter (8 Ft)	3.0 Meter (10 Ft)
Above 75 KV	2.5 Meter (8 Ft)	3.0 Meter (10 Ft)	3.7 Meter (12 Ft)

 Clearance around an Outdoor electrical panel (NES 110.31):

Description of Clearance	Distance  (min)
Clear work space:	Not less than 2.0 Meter high(Measured vertically from the floor or platform) or not less than 914 mm (3 ft) wide (Measured parallel to the equipment).
Entrance For Panel (More than 1200 Amp and over 1.8 m Wide)	One entrance required for working space (Not less than 610 mm wide and 2.0 m high )
Entrance For Panel: On Large panels exceeding 1.8 Meter in width	One Entrance at each end of the equipment.
Nonmetallic or Metal-enclosed Panel in general public and the bottom of the enclosure is less than 2.5 m (8 ft) above the floor or grade level	Enclosure door or hinged cover shall be kept locked.

 Elevation of Unguarded Live Parts above Working Space (NES 110.34E):

Voltage	Elevation  (min)
600 V to 7.5 KV	2.8 Meter
7.5 K V to 35  KV	2.9 Meter
Above 35 KV	2.9 Meter + 9.5 mm/KV

 Working Space for Panel (Code Georgia Power Company):

Voltage	Exposed live parts to Not live parts( or grounded parts )	Exposed live parts to Grounded parts (concrete, brick, and walls).	Exposed live parts on both sides
Up to 150 V	3.0 Meter	3.0 Meter	3.0 Meter
150 V to 600 V	3.0 Meter	3.5 Meter	4.0 Meter
600 V to 2.5 KV	3.0 Meter	4.0 Meter	5.0 Meter
2.5 KV to 9 KV	3.0 Meter	5.0 Meter	6.0 Meter
9 KV to 25 KV	5.0 Meter	6.0 Meter	6.0 Meter