Electrical Maintenance Testing Energized vs De-Energized Low and Medium Voltage Switchboards

Part 1 – De-Energized Maintenance

Many, if not all of our clients are reluctant to execute full or partial power outages for reasons such as; fear that the system will not re-energize sensitive operations that do not tolerate power outages, production schedules, cumbersome approval and sign off procedures or a basic lack of understanding. Clients frequently ask “Why we do have to de-energize? Can’t this be done hot?” The answer isn’t “No”, its Hell no! The NFPA 70E states that hot work will only be performed when de-energizing the system will create other hazards such as disabling ventilation or life support systems. In the next few paragraphs, we will discuss the various tests and evaluations that can be performed on the de-energized system. They all have merit; we will present the facts as we know them and let you draw your own conclusions.

Basics

Let’s start with the basics. An electrical system is energized when connected to any power source including any of the following; utility power, local generator power (including renewable energy such as wind and solar) or a UPS system. An electrical system is de-energized when disconnected from any and all power sources, properly locked out, and in many cases safety grounding instruments applied. All electrical systems are assumed to be energized until proven to be in a zero energy state.

Infrared Survey

The most common tool used for energized maintenance is the Infrared Survey. Today’s infrared cameras are relatively inexpensive, lightweight, and easy to use. FLIR and Fluke are the two most common on the market but there are many others.

Infrared Imaging is used to identify “hot spots” in an electrical system caused by higher than normal resistance, resulting in higher current flow and heat. We have avoided many potential catastrophic failures by addressing “hot spots” identified using an infrared camera. Be aware, however, you may have arc flash safety hazards that prevent exposing personnel to energized equipment. If your switchgear is not properly labeled identifying the Arc Flash Hazard Level, consult a qualified Electrical Engineer to assess the level of risk before opening any switchgear.

A comprehensive Infrared Survey program is a must for any facility large or small. Think of it like an annual physical exam for your electrical system.

Millivolt Drop Testing

Millivolt Drop Testing is a not so common, yet still used Predictive Maintenance practice. The millivolt drop test is performed between the line and load side poles of a breaker. Energized millivolt drop testing has been the subject of much controversial debate. Some Electrical Preventive Maintenance contractors have decided to use this method to advertise to customers that there is no need to shutdown equipment to test breakers. They proclaim it as an absolute test to assess the electrical integrity of connections and contacts in a circuit breaker. But NEMA (National Electrical Manufacturer Association) does not agree it should be absolute and writes in its recommended procedures the breaker should be de-energized and removed from enclosure before testing.

NEMA states in Section 5.4.1 of NEMA AB 4-(Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers used in Commercial and Industrial Applications) “The millivolt drop of a circuit breaker pole can vary significantly due to inherent variability in the extremely low resistance of the electrical contacts and connectors. Such variations do not necessarily predict unacceptable performance and should not be used as the sole criteria for determination of acceptability.

The millivolt drops test can show a great deal of variability and the variations do not necessarily predict unacceptable performance. The Molded Case Circuit Breaker standard, UL 489, for example, does not dictate particular millivolt drop values. UL 489 deals with temperature of the terminals, and other external breaker locations, under continuous current conditions. Thus the millivolt drop test should only be considered as an indicator of the circuit breaker’s thermal performance. It is not an absolute test.”

Another valid argument would be, “What is the load on the breaker when testing energized?” The answer is “When millivolt testing de-energized a 100% load is being used”. Most loads on breakers are between 40-80%. In this case, testing de-energized will catch anomalies not found under lower loads.

Ultrasonic Monitoring

Ultrasonic Monitoring is gaining in popularity in the electrical maintenance industry. When cable joints, switch contacts etc. are compromised; arcing, tracking and corona can occur, which produces noise in mostly inaudible wavelengths. Arcing, tracking and corona emissions produce ionization of the air, which in turn can cause corrosion and reduction in dielectrics that eventually lead to a flashover. When a flashover occurs the results can be devastating and extremely costly. Ultrasonic monitoring equipment is relatively inexpensive and easy to use. It is important to interpret the data gathered accurately to avoid chasing problems that don’t exist.

Partial Discharge

Partial Discharge (PD) is an electrical discharge that partially bridges the insulation gap between insulation and conducting electrodes of systems operating at 3000V and above, however, PD can occur at lower voltages. PD occurs when insulation properties have been compromised by age, damage, poor workmanship or design flaws. This testing can be applied to rotating equipment, metal clad switchgear, transformers, and cables. If ignored these conditions can result in a premature failure and leave you with expensive repairs. The flaws associated with PD tend to develop slowly over time so it is important to establish an early baseline and continue with regularly scheduled monitoring to track variations from baseline. PD equipment is expensive and requires a lot of training to use and interpret results. In most cases, an Electrical Engineer, familiar with PD monitoring will develop the test plan and interpret the results. Although this is an energized test, for safety reasons, quite often the system must be de-energized to install the detection probes, reenergized for the testing and subsequently de-energized to remove the probes.

Protective Relays

Protective Relays protect your most critical electrical systems so they deserve some attention. Protective relays come in three basic types. Electromechanical Relays are the “older” style relays, usually having one relay per phase. Solid State Relays are just as the name implies, solid state units and usually one relay monitors all three phases of the system. The Latest Generation Relays are all microprocessor based which really means you have a small computer watching your system and doing what it is programmed to do.

The approach to energized testing is different for each of these relay types. Relay testing requires injecting simulated faults, or other inputs, into the relay and recording the devices’ reaction to the input. Sometimes this requires lifting wires off the back of the relay. This isn’t something you want to do on an energized system unless you can tolerate a power outage. Some systems are designed to isolate the relay, one phase at a time or all three phases, for testing. While this is safer and less likely to cause a power outage, the system is left unprotected during the testing. This is where the older single phase relays have an advantage. Many are “draw out” relays which mean you can pull the relay out of the switchboard and test one phase at a time while maintaining protection on the other two phases.

We suggest you contact a qualified electrical testing contractor and have them explain the risks associated with relay testing on an energized system, and if they tell you “no problem, nothing to worry about”, find another contractor. With careful planning, many relays can be tested with your electrical system energized with little or no risk. For more info please go to our testing page.

By | 2017-11-06T20:59:15+00:00 December 4th, 2016|Maintenance|0 Comments