0560946064 Cutting Machine Grounding

Cutting Machine Grounding

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General Guidelines

Article Number: 0560946064Revision Date: January 7, 2011Revision Number: 00Language: ENG

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The purchaser is responsible for the safe operation and use of all products purchased, including compliance with all applicable standards in the country of use. See standard ESAB terms and conditions of sale for a specific statement of ESAB’s responsibilities and limitations on liability.

Information in this document is subject to change without notice. This manual is for the convenience and use of the cutting machine purchaser. It is not a contract or any obligation on the part of ESAB Cutting Systems.

ESAB Cutting Systems, 2010

Revision History:

00 - Original Release•

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Table of Contents

Introduction .......................................................................................................................................................................................... 5

Grounding Overview ......................................................................................................................................................................... 6

Basic Layout .................................................................................................................................................................................... 7

Elements of a Ground System......................................................................................................................................................... 8

Plasma Current Return Path ...................................................................................................................................................... 9

Plasma System Safety Ground................................................................................................................................................ 10

Cutting Machine Chassis Ground .......................................................................................................................................... 13

Rail System Safety Ground ...................................................................................................................................................... 14

Earth Ground Rod ............................................................................................................................................................................. 15

Ground Rod................................................................................................................................................................................... 15

Soil Resistivity ............................................................................................................................................................................... 15

Utility Power Electrical Ground .............................................................................................................................................. 16

Electrolytic Ground Rods ......................................................................................................................................................... 17

Multiple Ground Rods ............................................................................................................................................................... 18

Earth Ground Test ............................................................................................................................................................................. 19

Ground Cable Sizes ........................................................................................................................................................................... 21

Machine Grounding Schematic....................................................................................................................................................22

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MACHINE GROUNDING

5Introduction

Introduction

The information in this booklet is designed to help prepare for the installation of an ESAB cutting machine. Machine grounding is an important part of the installation process, which can be greatly simplified if prepared in advance. The most difficult part of the grounding process is designing and installing a low impedance Earth ground rod. However, the better the Earth ground rod, the less chance there is of having electromagnetic interference problems after the installation is complete.

Most national electric codes address grounding for the purpose of fire prevention and short circuit protection; they do not address equipment protection and electromagnetic interference noise reduction. Therefore, this manual presents more stringent requirements for machine grounding.

WARNINGElEctRIc Shock hAzARd.

Improper grounding can cause severe injury or death.

Improper grounding can damage machine electrical components.

Machine must be properly grounded before putting it into service.

the cutting table must be connected to machine earth grounding rod.

Grounding Overview

MACHINE GROUNDING

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Grounding Overview

There are two parts to a ground system;

Component or "chassis" ground•Earth ground•

Component grounding connects all pieces to a single component, like the machine chassis, which is then connected to a common point know as the star point. This provides a safe current path in case of a fault.

An earth ground provides a safe path for fault current and electromagnetic interference (EMI) to return to its source. Without a properly grounded system, an unintended path through people or sensitive equipment may be found, resulting in serious injury, death, and/or premature equipment failure.

This manual focuses on machines with a plasma cutting system. Machines with plasma cutting capability are particularly prone to electromagnetic interference problems and often utilize dangerous voltages and currents. All machines must have electrical components grounded and attached to an earth ground, regardless of process type (shape cutting, marking, or other material preparation).

A common symbol used to identify an earth ground on drawings.

A common symbol used to identify a chassis ground on drawings.

MACHINE GROUNDING

7Grounding Overview

Note: The three phase electrical input to the plasma power supply must include an electrical ground.

The electrical ground layout is similar for both large and small machines. The chassis ground , plasma positive electrical lead and the rail ground cables are attached to a common point on the cutting table. This common connection is referred to as a star point (see illustration below). One cable connects the star point to the Earth ground rod . The size of ground cables is dependant on the maximum current output of the plasma power supply . Specification of cable sizes is discussed later in this manual. Some country standards or directives require a separate ground rod for the plasma power supply. Consult your machine schematics for more information.

Basic Layout

This illustration demonstrates multiple ground cables fastened with a single bolt to create a star point . The location of the star point on the cutting table will vary.

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Elements of a Ground System

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The ground system consists of five main components:

plasma current return path •plasma system safety ground •utility power electrical ground •cutting machine chassis ground •rail system safety ground. •

Ensure provisions are made during the installation for each of these elements for creating a complete ground system.

MACHINE GROUNDING

9Elements of a Ground System

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Plasma Current Return Path

The return path ground cable is the most important element of the ground system. It completes the path for the plasma current. Solid, low impedance, well maintained electrical connections are a necessity.

The plasma cutting current is generated by the plasma power supply . A welding cable carries this current from the negative (-) connection in the plasma power supply through the x axis cable chain to the torch. The current then arcs to the work piece on the cutting table. The current path must be closed so that the current can easily return to its source. This is done by connecting the cutting table to the positive (+) connection on the plasma power supply. If the return path ground cable is not connected, the plasma system will not work. There will be no way for the arc to establish between the torch and the work piece. If the cable is connected, but the connections have a very high resistance, it will limit the current of the arc, and cause dangerous voltage levels between system components.


MACHINE GROUNDING

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The only way to ensure that all components are at the same voltage level (same potential), and thus eliminate the possibility of being shocked, is to ensure that all interconnections are making good electrical contact. Good electrical contact requires that connections are made with bare metal to metal contact, the connections are very tight, and are protected from rust and corrosion. Use a grinder or wire wheel to clean all paint, rust, and dirt from the surface when connecting cable lugs to any metal surface. Use an electrical joint compound between cable lugs and metal surfaces to prevent future rust and corrosion. Use the largest size bolts, nuts, and washers possible, and tighten fully. Use lock washers to ensure that connections stay tight.

Plasma System Safety Ground

The plasma system safety ground (or ground rod) serves several important purposes. It provides:

Frame voltage for personnel safety by ensuring •that there are no potential differences between system components and building components.

A stable signal reference for all digital and analog •electrical signals on the cutting machine.

Helps control electromagnetic Interference (or •EMI).

Provides a discharge path for short circuits and •high voltage spikes, such as those caused by lightening strikes.

MACHINE GROUNDING


Misconception about Earth ground rods.

There are many misconceptions about the ground rod, and the role it plays in reducing electromagnetic interference. In theory, the ground rod is present to eliminate possible potential differences between equipment and building structures. However many people believe that the ground rod allows all radio frequency noise to be absorbed and disappear into the Earth. Experience has shown that a good ground rod will eliminate radio frequency noise problems.

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In reality the ground rod is providing a low impedance path by which noise currents may return to their source .

Earth ground rod reality.

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MACHINE GROUNDING


Cutting Machine Chassis Ground

Since all electrical enclosures and shields on the cutting machine are connected to the chassis, proper function of the electronic systems on the cutting machine depends on the chassis being grounded. The cutting machine chassis ground connects the chassis of the cutting machine gantry to the plasma system star ground point. This usually consists of a 13.3mm2 [6 gauge] stranded copper wire, connected to the cutting table. This wire connects all electrical and chassis grounds on the machine to the ground rod. This wire is supplied with the cutting machine and will be connected during machine installation.


MACHINE GROUNDING

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Rail System Safety Ground

The rail system safety ground makes sure that the entire rail is at ground potential, eliminating any possible shock hazard, and providing backup for the machine chassis ground in case of a plasma current short circuit. All four corners of the rail system should be connected to the cutting table.

MACHINE GROUNDING

15Earth Ground Rod

Earth Ground Rod

The best way to make sure that your Earth ground connection is optimized is to enlist the services of a professional. There are a number of engineering firms which specializes in designing and installing Earth grounding systems. However, if this option cannot be used, then there are several things which can be done to ensure that your Earth ground connection is good:

Ground Rod

The ground rod itself can be optimized in two ways: length and diameter. The longer the grounding rod, the better the connection. The same is true for diameter: the larger the diameter, the better the connection. However, if the soil resistance is very low, then a ground rod longer than 3m [10 feet] does not make a significant difference. Since soil resistivity is rarely as good as it could be, a standard grounding rod should be 25mm [1 inch] in diameter and 6m [20 feet] long.

Soil Resistivity

Soil resistivity can be changed in two ways: by altering the mineral content, the moisture content, or both. The ideal solution to poor soil resistivity is to excavate the immediate area and backfill with conditioned soil additives. In extremely dry areas, the moisture content can be improved by installing a drip system which continually moisturizes the soil surrounding the ground rod. A crude way of affecting soil moisture and content is to use salt water, or rock salt to condition the surrounding soil.

Earth Ground Rod

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Utility Power Electrical Ground

The utility power electrical ground must accompany all 3 phase and single phase power feeds. This electrical ground provides the proper reference for all incoming power. Failure to provide this ground is a violation of most electrical codes, and a serious safety hazard.

Depending on the 3 phase power arrangement (either a “Delta” or a “Y”), the line to ground voltage may be equal to, or less than the line to line voltage. A problem exists any time the line to ground voltage exceeds any individual line to line voltage (difference in potential). Contact your local utility company if you are not sure that your 3 phase power has a proper electrical ground. Make sure that your electrical contractor properly installs the electrical ground wire with all 3 phase and single phase power feeds.

The electrical ground must be connected to the appropriate terminal inside of the plasma power supply. Size wire according to local electrical codes.

Utility Power Electrical Ground

3 Phase Electrical Supply

Plasma Power Supply

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MACHINE GROUNDING

17Earth Ground Rod

Electrolytic Ground Rods

One solution that may be suggested by a grounding expert is the use of an electrolytic ground rod, with conditioned backfill. This option can be expensive, but will give the best possible ground connection. To install one of these rods, the ground must be excavated or drilled, the rod installed, then conditioned soil used to backfill around the rod. The result is a very low impedance earth ground, which will maintain itself for the lifetime of the cutting machine. If the concrete slab on which your cutting machine will be installed has not yet been poured, then the electrolytic ground rod may be the best option for your grounding system.

Earth Ground Rod

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Multiple Ground Rods

There are a number of reasons why multiple ground rods should not be used. While installing multiple rods may improve a safety ground or lightening ground, it offers no advantage for electromagnetic interference reduction, and can cause more problems than it is worth.

The problem with multiple ground rods is that each rod uses an “interfacing Electromagnetic Interference sphere” of earth, having a radius of 1.1 times the length of the rod. Overlapping of these Electromagnetic Interference spheres causes a loss in grounding effectiveness proportional to the amount of overlap.

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Multiple ground points can also create undetectable “sneak” pathways for radio frequency noise currents, actually causing more interference! Instead of considering multiple ground rods, take steps to make the single ground rod as good a ground connection as possible.

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Multiple ground rods should be avoided if possible. However, if all other avenues have been explored to lessen your systems’ electronic interferences, multiple ground rods are an option.

Such a system should be installed by a professional and the distance between the rods should exceed 2.5 times the length of the rods.

MACHINE GROUNDING

19Earth Ground Test

Earth Ground Test

An Earth ground test should be conducted to test the impedance of your system. A specialized tester can be used. If necessary, contact a specialist to conduct the test.

It is possible to create your own test circuit in North America. This method may not be appropriate, or even illegal in some countries. Check with local/national codes or ordinances before proceeding.

Earth Ground Test

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115 VAC HOT

110w light bulb

Ground rod

Voltmeter set to measure AC

AC NEUTRAL

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Connect a 110 watt light bulb between standard AC current (115 VAC HOT) and the cutting machine’s grounding rod. Then connect a digital voltmeter between AC NEUTRAL (from the same source) and the grounding rod.

The meter will display the voltage between AC NEUTRAL and the ground rod, which is equal to the resistance in ohms between these two points.

The ideal condition is 3 ohms or less between ground rod and electrical ground. However, this value may be difficult to achieve. To reduce the ground resistance, one of the following steps can be taken:

Increase rod length and/or diameter.•Condition the soil surrounding the ground rod •by adding moisture and salt.

Use an electrolytic ground rod with conditioned •backfill.

WARNINGElEctRIcAl Shock hAzARd

Under certain situations a dangerous hazard may exist if anyone may contact the ground rod during testing.

do not touch the ground rod while power is being applied to the light bulb .

MACHINE GROUNDING

21Ground Cable Sizes

Ground Cable Sizes

Cable sizes must be able to handle the largest possible current fault on a machine. Since a plasma torch usually requires the largest current, cable sizes are dependent upon the size of plasma system. As a general rule of thumb, plasma ground cables should be at least half the size of plasma power cables. A machine with only oxyfuel process might require a star-to-rod cable with a size of 35mm2 [2 AWG]. The same machine with an ESAB EPP-360 plasma power supply and PT-36 torch will require a star-to-rod cable with a cross-sectional area of 70mm2 [00 AWG]. Check with an ESAB representative for more information on ground cable size specifications.

Machine Grounding Schematic

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Machine Grounding Schematic

Main Control Enclosure

Component Enclosures

Maine Star Ground

Rails

Cutting Table

System Star Ground (on Table)

Earth Ground Rod

Plasma Power Supply

Plasma Power Supply Ground (required by EU Standards)

Electrical System Ground

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All electrical enclosures bolted to the •machine chassis

Machine chassis grounded to star point on •cutting table.

Rails grounded to cutting table•Plasma ground connected to star point on •cutting table

Earth ground rod connected to star point on •cutting table.

A separate ground rod is required for the •plasma power supply by some regulations and directives. Check with local regulations to determine if this additional ground rod is required.

MACHINE GROUNDING

23Machine Grounding Schematic

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Customer // Technical Support

(843) 664-4405

(800) ESAB-123 (372-2123)

ESAB Welding and Cutting Products

PO BOX 100545 Ebenezer Road

Florence, SC 29501-0545

http://www.esab.com

ESAB Cutting Systems – Canada

6010 Tomken Road

Mississauga, Ontario Canada L5T 1X9

Phone: (905) 670-0220

Fax: (905) 670-4879

ESAB Cutting Systems GMBH

Robert-Bosch-Strasse 20

Postfach 1128

D-61184 Karben 1

Phone +49-6039-400

Fax +49-6039-403-02

http://www.esab.de

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0560946064 Cutting Machine Grounding