1. Using Clamp Meters in Residential Applications
For residential utility electricians, a clamp meter is an essential tool for measuring. While a spot check of the current is often sufficient, sometimes this check does not provide a complete picture as the load is turned on and off, over several cycles, etc.
Voltage in an electrical system should be stable, but current varies widely. To check for peak or worst-case loads on a circuit, use a clamp meter with a min/max function designed for measuring high currents that exist for longer than 100 ms or about 8 cycles . These currents can cause intermittent overload conditions that can cause annoying tripping of the circuit breaker.
The load on each branch circuit is measured at the switchboard The measurement is made on the load side of a circuit breaker or fuse that will open the circuit in the event of an accidental short circuit, which is especially important for any type of direct contact voltage measurement , even if the clamps of the clamp meter are insulated, due to the fact that there is a level of protection not available for direct contact voltage measurement, care is necessary.
A common problem in electrical work in residential facilities is mapping electrical outlets to circuit breakers, and clamp meters are useful in identifying which circuit a particular outlet is on. First, take a baseline reading of the current current of the circuit on the panel. Then, put the clamp meter in max/max mode. Go to the relevant outlet, plug in a load (a hair dryer is ideal), and turn it on for a minute or two. Check that the maximum current reading of the clamp meter has not changed, a hair dryer will typically draw 10-13 A, so there should be a noticeable difference. If the readings are the same, the circuit breaker is incorrect.
2. Using clamp meters in commercial applications
Clamp meters are used to measure circuit loads on feeders and branch circuits at switchboards. If there is a circuit breaker or fuse, the measurement of the feeder should always be made on the load side of the circuit breaker or fuse (eg in an enclosed motor starter).
The balance and load conditions of the feeder cables should be checked: the currents on all three phases should be approximately the same to minimize currents returning to neutral.
The neutral line should also be checked for overload. With harmonic loads, even if the feeder phases are balanced, the neutral conductor may carry more current than the feeder. Each branch circuit should also be checked for possible overloading.
Finally, ground loops should be checked. Ideally, there should be no current in the ground return, although less than 300 mA is normal.
3. Clamp meter to test leakage current
To check for leakage current on a branch circuit, place the live and neutral wires in the jaws of the clamp meter. Any current measured is leakage current, that is, current returning to the ground return. Supply current (black line) and return current (white line) generate opposing magnetic fields. The currents should be equal (and opposite) and the opposing magnetic fields should cancel each other out. If there is no cancellation, it means that some current (called leakage current) is returning from another path, and the only other path is the ground return.
If you detect a net current between the supply current and the return current, you need to consider the nature of the load and circuit. A miswired circuit can cause up to half the total load current to flow through the grounded system. If the measured current is very high, there is most likely a wiring problem. Leakage current can also be caused by load leakage or poor insulation.
Worn windings in the motor or moisture in the clamping mechanism are common culprits. If you suspect a leak, a power outage test with a megohmmeter will help assess the integrity of the circuit insulation and help determine if and where the problem is.
Fourth, the clamp meter measures each load
To measure individual loads, a pinout can be used at the socket. It is simply an extended cable with the outer insulation stripped to expose the black, white and green wires. This is much easier than pulling the socket out to access the wires. Plug the load onto the cable and plug the cable into the socket. To measure load current, clamp the black wire. Perform a ground current check directly on the green wire or on the black wire along with the white wire.
Five, clamp meter test motor and motor control circuit
One of the most challenging places to make current measurements is in the control cabinet, especially when it uses IEC type components. IEC-type parts originating in Europe are much more compact than the corresponding NEMA parts, and the wiring may be packed quite tightly. Fluke 330 Series Clamps drive fan and pump loads. The motor can be controlled by an electromechanical starter, or by an electronic variable speed drive. Variable speed drives are becoming more common because they save a lot of energy.
The Fluke 337 is the ideal clamp meter for the following motor and drive measurements:
Load size: The motor current sinking, measured as the average of the three phases, should not exceed the motor's full load current rating. The tapered clamp and backlighting features of the table are ideal for this measurement task. Multiply by the allowable overload factor'
On the other hand, motors with load currents below 60% of full load current (as they are in most cases) become less efficient and their power factor drops.
Current balance: Current unbalance can indicate a problem with the motor windings (for example, different resistances on the field winding due to internal short circuits (generally, the unbalance should be less than 10 %). (To calculate the unbalance, first calculate the three-phase Average of the readings; then find the maximum deviation from the average and divide by the average.) Extremely high current unbalance is a single-phase unbalance when there is no current in one of the three phases. This is usually caused by an open fuse cause.
Inrush current: Motors that are directly pressurized (via a mechanical starter) have an inrush current (the inrush current can be around 500 % on older motors and up to 1200 % on energy-efficient motors. Too high an inrush current will often Causes voltage dips and annoying tripping. The Fluke 337 Clamp Meter's "Inrush Current" feature is a unique feature designed to trigger on an inrush current and capture its true value.
Peak loads (shock loads): Some motors experience shock loads that can cause enough current surges to trip the overload circuit in the motor controller.
Imagine running into a hard knotted chainsaw. The minimum/maximum function can be used to record the worst-case current absorbed by the shock load.
Clamp meters are indispensable tools for electricians, whether in residential or commercial facilities.
