Isolation of Electrical Energy Guidance

Transcription

1 Page 1 of 8 i ii Isolation of Electrical Energy Guidance 1. Scope 1.1. This document provides guidance on the various methods of isolation of electrical energy and highlights some (not all) of the limitations of these methods and the equipment typically in service at Dow facilities. These guidelines DO NOT address the training requirements, skill requirements, electric shock hazards, arc flash hazards or PPE requirements to perform any task or activity mentioned herein. For electrical safety requirements refer to the Global Electrical Safe Work Standard This document will also address Isolation of Electrical Energy where electrical energy is converted to mechanical energy and where the mechanical energy is of the primary concern. The typical example here is an electric motor application where the driven equipment or mechanical equipment requires the isolation i.e. electrical work is not being performed Other parts of the document will also provide additional guidance where an electrical task or work is required. In all cases, the Isolation of Energy Standard (IEOS) applies. 2. Introduction 2.1. First and foremost when isolating energy on any system or piece of equipment, practice Leadership and Engagement as follows: 2.2. Make it Personal Know what you are isolating and why you are isolating it Review the Safety Electrical One-Line Diagram (SEOLD) and/or equipment schedules. Look for the electrical sources and isolation devices to be used. Equipment labels and SEOLD equipment labels need to be the same. Look at the one-line diagram and make sure the drawing has been recently updated before initiating any isolation activity If you have a visible air gap or other confirmed isolation, stop and think about what could lead to the sudden disappearance of the air gap or loss of that isolation (stored energy devices, some parts of control power remaining that can reactivate the circuit, etc). Note that confirmation that an isolation switch is off or even if there is a visible air gap is not the same thing as confirmation of isolation. Test before touch is required for electrical work and a try test or observance of a visible gap local to the equipment is required for mechanical work. Do not proceed until you are absolutely confident that the isolation is confirmed and cannot be compromised (Prevention of compromise of the isolation is typically accomplished by mechanical restraint or by rendering the operator non-functional) Ask open ended questions Ask the person you are isolating this equipment for, what they are going to be doing, and how they will be doing the work safely. Ask probing, open ended, questions such that the person must clearly explain their

2 Page 2 of 8 work and boundaries. If you do not feel they understand what they are doing, stop the job, assess and escalate as appropriate Create and foster and environment where everyone can ask questions. When you are isolating a system or piece of equipment, encourage questions from the person you are isolating it for. Create an environment where the person feels comfortable asking you such questions about how you are going to clear the equipment for their safety Escalate If there is anything you do not feel comfortable in isolating, then stop and escalate. If you do not trust the one-line diagram, you feel rushed, feel uncertain about what you are doing, or you feel the individual you are performing the isolation for is rushed, uncertain, etc., then stop and escalate. 3. Identifying the equipment 3.1. The most critical item when dealing with electrical energy isolation is correct equipment identification The electrical isolation device (switch, circuit breaker etc) and the associated equipment (load) being isolated must have unique and easily reconcilable identifiers linking their operation. If at any time, the labeling of the equipment is ambiguous or the identification of the equipment is unknown, stop the isolation process until you have a clear understanding. Refer to EMETL G7M to determine the isolation device identification Avoid the use of the same or confusing equipment identifications within the facility. When performing and validating the isolation be sure to read equipment labels/ids/tag Numbers carefully, write them down and compare both the ID of the isolation device and the equipment/circuit being isolated. As required by the IOES standard at least two people (typically this is the Facility Representative and the Job Representative), must agree on the equipment identification when performing and validating the isolation. If needed, follow the MOC process and label equipment with unique equipment identifiers Knowledge of key interlock (Kirk Key or similar) systems. Some systems rely on these key interlock systems to force a particular sequence of operation as part of isolating electrical equipment. Procedures must exist and shall be followed explicitly when operating these key interlock systems. If you do not have a procedure or do not understand what the procedure is indicating, stop and escalate. 4. De-energize, Tag and Try of electrical energy sources 4.1. De-energize, Tag and Try has proven an acceptable and effective method of isolation of electrical energy for non-electrical work. However, understanding the limitations outlined in the following, is important: 4.2. Interlocks and control circuits cannot be the primary means of isolating any equipment. Mechanical isolation means must be used. This is a minimum IOES requirement Proper identification of the equipment is always required.

3 Page 3 of Be cautious and questioning when verifying de-energization, tag and try method after an equipment trip. Fuses can blow, overload relays can operate, circuit breakers can trip that will turn power off to the load. These devices must be reset or fuses replaced to actually validate the try step. Failure to do so, in conjunction with any equipment ID confusion may lead to improper isolation of electrical energy Equipment (typically motors) with computer/plc control may have permissives in control circuits that make it more difficult to perform the try test. Sometimes these systems may not have a field switch or the field switch function is Auto- Off only. In these circumstances, the following options can be considered: Install a field switch that has Hand (Jog) Off Auto position, where a spring loaded Hand(Jog) position wired to bypass permissives and computer control Install an isolation device (such as a motor safety switch or local motor disconnect) at the equipment. This allows for local visual inspection of isolation and better certainty that the equipment is isolated. Application of this method requires confirmation with appropriate SMEs and documentation that the local switch isolation is confirmed in this case to be the equivalent of a try test Local safety switches with factory installed viewing windows offer an effective method for external non-contact confirmation of a visible air gap when this gap is required for electrical isolation Where applicable, local isolation switches must meet the requirements of IEC Develop and use TRY procedures to identify the permissives (computer/plc inputs and outputs) that are to be overridden or place in manual as part of the try. This would be considered a critical procedure and will require consultation with plant process control focal points For non-electrical work, these key steps that must be included into the facility Try procedure/process for equipment that is currently operating: Two people (Facility Rep and Job Rep typically) are required to identify the equipment and associated isolated devices. The identification of isolation device, motor/equipment, field control switch, and or other controllers must match. Use your SEOLD, MCC data sheets, panel schedules, etc. as needed Use the identified field control switch or process control logic; stop the motor/equipment Isolate red tag and/or lock out the identified isolation device(s) Attempt to start the identified motor/equipment with the field switch or process control computer. Do not proceed if equipment starts. Contact an Electrical Skilled Person to help in identifying the correct isolation device (See G7M ) Some process equipment may not be capable of the try process due to material plugging or physical damage. Performing the electrical test before

4 Page 4 of 8 touch by an Electrically Skilled Person (ESP) or Electrically Instructed Person (EIP) is an accepted practice if all starter isolation devices are locked out / tagged out in accordance with IOES Some US Plants have installed specialty voltage indicating devices and have demonstrated effective alternate try procedures for verifying isolation of energy for mechanical work. For application assistance in this area, please contact a member of the GESMN. 5. Isolation Device Limitations 5.1. Isolation devices for electrical equipment are mechanical devices and are subject to failure. The operators for most isolation devices are located externally to the equipment. The actual isolation device is typically within the enclosure with a mechanical linkage between the external operator and the electrical contact part of the isolation device (typically a switch or circuit breaker) 5.2. Being familiar with the operation of these devices is essential. Use your senses to feel and hear the operation of the device. You should feel resistance when operating the device. You should hear and feel a click or noise when the switch/cb operates. If you are not sure how the devices should feel or sound, get with your Facility Electrical Representative (FER) and practice on a spare de-energized piece of equipment. You should suspect a mechanical failure of the operator if you do not get the usual feedback from the operation. The key message here is that just because the operator is in the off position does not mean that the circuit is off or isolated Likewise, do not trust your senses explicitly and assume the device operated properly when you hear the click or feel resistance. The switch or CB itself can also fail mechanically. For example: The device may break and open only 2 of the 3 phases. The circuit is still energized, but 3 phase motors will not turn under this condition The device may break and open only 1 of the 3 phases. The circuit is still energized, but 3 phase motors should not start from zero speed under this condition Another good indication that the isolation device operated is that indicator lights, meters, metering etc. should go off/out when the device is turned to the off position Some local isolation practices require confirmation of a visible air gap for electrical work. For NEMA/ANSI type equipment, open the equipment door (if not provided with a window) and inspect the CB or switch. The CB should be in the OFF position or all knife blades or contacts of the switch should be visibly open i.e. air gap. Caution! Opening electrical equipment doors may be energized electrical work. Additional PPE may be required and ESP/EIP qualifications are required For IEC low voltage MCC s, the blades or contacts of the switch cannot normally be seen. Typically, a mechanical indicator of the breaker/switch position is visible from outside the cabinet.

5 Page 5 of After the internal visual inspection, close the door and apply locks/red tags as needed. If a deficiency is found contact your Electrical Skilled Person (ESP) for resolution/repairs Some low voltage MCCs are designed with the isolation device handle mounted to the enclosure door. Once the isolation device has been switched off and locked in the off position, the door is mechanically held captive and cannot be opened. For work in this enclosure, a red tag cannot be applied and the work must be performed as Energized Electrical Work. If the work location needs to be left unattended for any reason, the door must be secured, the circuit deenergized and Red Tagged by operations Other LV MCCs are also designed with the isolation device handle mounted on the enclosure door but can be opened when switched off by defeating the mechanical interlock. For this type of equipment, opening the door while Red Tagged is considered a violation of the IOE Standard. If work is required inside the enclosure the following two options are allowed: Use the Red Tag addendum process to temporarily remove the tag or Reconcile the red tag and use the Electrical safe Work Standards to complete the task 5.9. Many LV panel boards have a dead front guarding that protects a worker from contacting exposed and energized components. Some of these panels also have breaker locking mechanisms that are integral to this cover. If a single or multiple breakers are Red Tagged and locked using a device that relies on the cover being in place, removing this cover is a violation of the IEO Standard. The only three options are: Use the red tag addendum process to temporarily remove and relocate the tag or Reconcile the locks and tags and use the Electrical Safe Work Standards to complete the work If the breakers are Red tagged only, and the tag can slip through the breaker opening in the cover, the cover can be removed as long as the circuit that remains tagged is not disturbed and the tag remains visible. 6. Molded Case Circuit Breakers (MCCB) 6.1. MCCBs are totally enclosed, often non removable and one cannot see the electrical contacts within. They are typically utilized at low voltages (up to 690V) principally as a short circuit and/or over-current protection means. In some cases, depending on the design standard the MCCB complies with, it can be suitably used as an isolating device (e.g. if the device is recognized as suitable for isolation in accordance with IEC or other industry standards) MCCBs being enclosed, as such the try part of De-energize, Tag and Try and ID of the equipment is critical for non-electrical work For electrical work behind an MCCB, some EIP or ESPs may require establishment of an air gap, particularly at voltages above 300 volts. This typically requires load side cable removal, which may be undesirable for several reasons. Some alternates to this are discussed below:

6 Page 6 of For electrical work behind a MCCB, one potential alternative to establishment of an air gap is to attach a Personnel Protective Ground (PPG) per the strict requirements in EWP-33 on the load circuit. The PPG can be thought of as similar to a bleed valve on a piping system and will divert hazardous electrical energy to ground in the event of inadvertent reenergization. Any PPG used in this fashion must be red tagged since it is being used as an isolation device. Attachment of PPGs to circuits downstream of smaller MCCBs, in many cases, is impractical The isolation device upstream of the MCCB may be used if any doubt exists on proper function of the MCCB. This may also cause electrical outages to other equipment A MCCB in a control panel will sometimes feed a series of fuses/fuse blocks for distribution of associated circuits within or external to the panel. In these cases, removal of these fuses can be applied to establish an air gap After operating the MCCB to the Open position, the load side of the MCCB can be tested by an ESP or EIP to verify voltage is not present and that the breaker operated properly Some MCC buckets using circuit breakers as the isolating device are designed to be partially withdrawn from the power bus and locked in a neutral position. This prevents the stabs from contacting the bus and assures a safe, but mostly invisible air gap. 7. Variable Frequency Drives 7.1. Variable frequency drives are frequently controlled by DCS or panel mounted Loop Controllers. In this control scheme, a speed setpoint is provided to the controller either automatically or manually. The output is controlled based on the selected mode of the controller. When placed in automatic mode, the controller will adjust the output to the VFD to satisfy the speed setpoint based on the speed feedback designed into the controller. When in manual, the speed output is manually entered. Many DCS programs will set the speed to zero to initialize the VFD when not in use. This makes the try step in IOE difficult because the controller output must be known and set to a sufficient speed to observe motor shaft movement The VFD may also lack the typical field stop / start or stop / jog pushbuttons and speed control potentiometer. Lacking local controls for the verify step of IOE requires the use of reviewed verification procedures. These can guide the authorized procedure user through the proper steps to assure the correct energy isolation device was operated Bypass contactors VFDs can be installed with separately powered bypass contactors to increase the driven equipment reliability in the event of a VFD failure. The bypass contactor allows the motor to operate at rated motor speed by running at full supply voltage and frequency. When isolating this type of installation, both power supply energy sources must be tagged and verified to be de-energized.

7 Page 7 of 8 8. Electrical Work 8.1. Only EIPs or ESPs may perform energized electrical work Test each conductor before touch! Regardless of method or approach used to isolate electrical energy, one must test (with an approved meter) each and every conductor before touching them in accordance with the Global Electrical Safe Work Standard requirements. 9. Safety Notes 9.1. In many cases the line (source) side of the isolation device can remain energized after operation of the isolation device to isolate the load side. Hazardous voltages can still exist within the enclosure and while in most cases these are guarded from inadvertent touch by means of barriers or shrouds it needs to be understood that guards can break or be unintentionally left off from previous maintenance activities. Understanding the fundamental design of the equipment is important to safe and reliable operation Multiple sources of power may be present inside enclosures and starters. Sources may include DC control voltages, auto transfer switches, and electrical interlock/control circuits from other equipment etc. This means that hazardous voltages may still exist within the equipment enclosure even when the main source of power has been safely isolated. Virtually every automatic transfer switchgear system will contain some control voltage within the switchgear cell even if the main power source has been disconnected from one of the sources. It is important and in some countries a legal requirement to label the equipment with a warning indicating that it has multiple/ alternate sources of electrical power. Text for an example warning would be This equipment has an alternate source of power and is fed from list circuit ID and isolation device. For switchgear systems, multiple sources of power are likely within a circuit breaker cell or within the equipment, such warning signs are not necessary but in all cases, the isolation and associated work permits must caution the worker that other sources of control power are likely to be found within the switchgear or circuit breaker cell These guidelines DO NOT address the electric shock or arc flash hazard PPE requirements to perform any task or activity mentioned herein. Refer to the Global Electrical Safe Work Standard as needed. 10. Summary (or Applying Layers of Protection) Dialog and questions Use of documents and drawings to include SEOLD, MCC data sheets, panel schedules, etc Use of electrical or other resources to assist (i.e. escalate) Accurate name plates and identification tags of equipment and equipment isolation devices.

8 Page 8 of Two people (Facility Rep and Job Rep) validate isolation device and equipment identifications. For any discrepancy use G7M to determine proper isolation device Operate isolation device and isolate in accordance with IOES Training knowledge and skills with the equipment involved. Use your human senses to hear, feel and see the isolation device operate Use of a TRY process or procedure. In some cases, these may be considered critical procedures. Do not proceed if equipment starts/runs Test for electrical energy isolation (test before touch) before any electrical work. Do not proceed if electrical energy is on. i ; MOC 2012_11300; By: Joe Beam; Supersedes Issue Date 19-JUN-2009; Major revisions to entire document including additions, deletions and clarifications throughout. Key areas of change include: Section 2.2.2: Improved guidance for reliance on visible air gaps. Section 4.5: Added information for installation of alternate IOES verification devices. Section 4.6: Added information for use of test before touch method. Added information of alternate IOES verification devices. Section 5 Added IOE guidance for MCCs that do not allow internal access when locked and red tagged. Added IOE guidance for MCCs that disconnect the tagged operating devices from the isolation device. Section 6.3.5: Added IOE paragraph for MCC buckets using circuit breakers as isolating devices. Added section 7 for Variable Frequency Drives isolation. ii 19-JUN-2009; MOC 2009_07439; By: Don Hull; New practice. This guidance document is provided by the Global Electrical Safety and Maintenance TRN in association with Personal Safety Expertise Isolation of Energy Standard Owner.