The first place to inspect when you experience power quality problems is your service panel. From voltage sags and tripping circuit breakers, to overheating panels and spikes in your power, your distribution board (DB) can help you pinpoint your problems. But where do you start? The specialists at electrical tool and equipment manufacturer Comtest have put together an easy-to-follow guide on the five most common trouble spots on your panel and how to fix these issues.
Depending on the voltage and measurement requirements, you can use various tools for power quality troubleshooting, from digital multimeters to handheld single- and three-phase power quality analysers that perform many calculations automatically.
1. Bad circuit breakers
Since circuit breakers are pieces of equipment, they have a lifespan that needs to be monitored. Breaker parts such as contact points and springs wear out, and the physical switch mechanism can break. This is one of the easiest parts of the DB board to replace, but you first need to find the one that is giving trouble. Measurements of the circuit breaker voltage drop help to determine the breaker’s overall condition. This is done by measuring across the line-to-load side of the branch breaker – if the voltage drop exceeds 100mV, the breaker should be replaced (it should be in the 35 to 100mV range).
2. Current balance and loading
To check the current balance and loading, measure each feeder phase and current on each branch circuit. It is critical that the correct tool is used to record the measurement: A true-Root Mean Square clamp or true-Root Mean Square digital multimeter with a clamp-on accessory. An average responding clamp-on meter will not provide an accurate measurement because the combination of fundamental and harmonic current makes this a distorted waveform. A lower-cost average-sensing meter will tend to read low, leading you to assume that the circuits are more lightly loaded than they are.
3. Grounding hotspots – loose connections or terminals
Neutral ground bonds in subpanels are a violation of the electrical code, as well as of power quality performance wiring, but they are also quite common. Neutral ground bonds should be made at the transformer, never downstream of the main panel. When a neutral ground bond is made at a subpanel or receptacle, the ground path becomes a parallel return path for normal load current, which results in measurable current on the ground.
When it comes to hotspots, poor connections and the resulting heat loss are the main reasons for system inefficiency, and loose terminations lead to excessive source impedance. They are quite easy to find by using an infra-red thermometer, such as the Fluke 60 Series from Comtest, which allows you to detect a hotspot without having to make physical contact with the panel.
4. Harmonics
To check for harmonics, you need to measure the current on the feeder neutral. This usually falls between the 80 to 130 percent range of the feeder current because the third harmonic will add up in the neutral.
5. Voltage level (steady state) and voltage stability (sags)
The best way to check your voltage level and stability is to measure voltage levels of the branch circuits, phase-to-neutral, at the load side of the branch circuit breakers. If levels are low at the breaker, they will be even lower at the receptacle. This could be caused by low tap settings at the transformer, or even loose connections, long feeder runs, and overloaded transformers that create excessively high source impedance.
If intermittent voltage sags are the suspected issue, you will need to begin at the panel to isolate the cause. Are the sags a result of loads on the same branch circuit, or are they caused by loads elsewhere in the system? Comtest’s Fluke Power Quality Analyser is a multi-channel recording instrument that can isolate the sag’s source by trending voltage and current simultaneously.
If a voltage sag occurs together with a current surge, the sag is being caused by a load on the branch circuit, which means that it was downstream of the measurement point and is considered a load-related disturbance. If the voltage sag coincides with a minor change in current, the sag has been caused by something upstream of the measurement point and is a source-related disturbance. If the sag is deep and approaches an outage, the source of the problem is more than likely going to be the electrical grid.
Enquiries: www.comtest.co.za
