Fluke Tool Guide – How To Use A Clamp Meter
Have you ever been stood in front of a stopped production line, desperately trying to work out what the problem is as everyone stands their tapping their feet and waiting for you to fix it? Or how about a simple circuit breaker that keeps on tripping and you can’t work out why?
A clamp meter is a fantastic tool – used to measure circuit loading, it can also have a variety of other useful functions such as working out which circuit breaker controls which outlets, or measuring individual loads for load and earth currents. With a good clamp meter and a little know how, this multi-functional tool can help you to solve problems quicker and maintain your reputation as an expert electrician.
How Clamp Meters Work
Put simply, there is only one practical way to measure the current on an electrical wiring system, by measuring the magnetic field that surrounds a current-carrying conductor, and a clamp meter allows you to do just that. The alternative is to break each circuit open to take individual circuit measurements which is both impractical and carries the risk of taking critical loads offline, causing numerous problems for your client!
With a clamp meter, readings are generally taken at the electrical panel and consist of loading and balance on three-phase feeders. Due to the prevalence of harmonic loads, it’s also necessary to take neutral measurements at panelboards.
Clamp meter current measurements also come in handy in diagnosing the health of a motor!
Features of Clamp Meters
Modern digital clamp meters, such as our premium-quality Fluke clamp meter range, can take not only the basic measurements expected of a clamp meter but also voltage and resistance readings. That makes this fantastic little tool a must-have as an electrician; in fact, you may find it is the only tool you need for taking everyday measurements on the job.
Average-sensing models are lower in price but do not take such accurate current readings, hence why we would recommend a true-rms model such as the Fluke 365 or the Fluke 373, for the professional electrical who requires reliable, exact readings. With an average-sensing model, the reading could be inaccurate should the circuit have electronic loads such as computers, TVs and lighting, with greater electronic loads leading to greater inaccuracy. A properly calibrated true-rsm clamp meter on the other hand will always be accurate.
A clamp meter is a great tool for residential electricians as it allows you to measure the individual loads of different branch circuits at the distribution panel.
A simple current ‘spot check’ is often insufficient as loads can be switching on and off or going through cycles – voltage should be stable in the electrical system, but current on the other hand can be pretty dynamic.
A clamp meter with a min/max function will allow you to check the peak or worse-case loading on the circuit by catching high currents that exist for longer than 100ms (approximately eight cycles). Such currents lead to intermittent overload conditions which can be the cause of that constantly tripping circuit breaker!
To take clamp meter measurements, read them on the load side of the circuit breaker or fuse – the circuit breaker will open in the event of an accidental shortcut. Measuring this way is all the more important in any kind of direct-contact voltage measurement. Clamp jaws are insulated for a high level of protection not available in direct-contact voltage measurements; however it is still a good idea to err on the side of caution.
A clamp meter can prove useful in identifying which circuit a particular outlet is on, allowing you to map each outlet to its corresponding circuit breakers, a common problem that comes up in residential electrical work.
To do this, take a baseline reading of the existing current on the circuit at the distribution panel. Next, swap your clamp meter to its min/max mode and plug a load (e.g. a hair dryer – it draws around 5a so there should be a noticeable difference in the reading) into the outlet in question and check the clamp meter. If the maximum current reading changes, you have got the right circuit breaker!
A clamp meter is equally useful when it comes to commercial electrical work. It can be used at the panelboard to measure circuit loading both on feeders and on branch circuits; branch circuit measurements should always be made at the load side of the circuit breaker or fuse.
Using a clamp meter to perform basic circuit checks:
- Feeder cables should be checked for both loading and balance, and current on all three phases should be around about the same, minimising the return current on the neutral.
- The neutral should also be checked for overloading as it is possible that with harmonic loads the neutral is actually carrying more current than a feeder, even if the feeders are balanced!
- Each individual branch circuit should also be checked for overloading.
- The earth circuit should be checked also (see below); ideally you want there to be no current here, although in certain installations a level of under 300mA is not a problem.
Earth Circuit Leakage Currents
Put the live and neutral wires of a branch circuit in the jaws of your clamp meter – any current measured is leakage current, which is current returning on the earth circuit.
This is caused when there is an imbalance between the supply and return currents, as their opposing magnetic fields that are being generated do not cancel each other out and hence some current is ‘leaking’ back on the only other available path, the earth current.
Your clamp meter can detect and measure this leakage current quickly and easily. If detected, consider the nature of the load and of the circuit to work out what the problem is. If the leakage current is very high it is likely caused by a wiring problem, which can lead to up to half of the total current going through the earth system!
However, poor insulation and ‘leaky’ loads can also lead to leakage current. A common culprit is a motor with either worn windings or moisturise in the fixtures. When an excessive current leakage is suspected, use a megohmmeter to carry out a de-energised test that evaluates the integrity of each circuit’s insulation and will help to identify if, and where, the problem exists.
Motor Control Circuits
A control circuit cabinet, especially if it uses IEC-style components, can be one of the most challenging places to make current measurements! European-originates IEC-style components are much more compact than their NEMA counterparts and you may find that the wiring is packed pretty tightly.
In such a situation, you are likely to find the Fluke 330 series extremely handy with its tapered jaw and backlight functions!
Commercial buildings commonly make use of three-phase induction motors to drive fan and pump loads, and these motors can be controlled by either electromechanical starters or by electronic variable speed drives, the latter being most common due to their considerably energy savings.
To take these motor and drive measurements, we recommend the Fluke 374 with proprietary inrush measurement technology that filters out noise to show you motor starting current exactly as circuit breakers and fuses see it.
The motor’s current draw (measured as an average of the three phases) should not exceed the full load amperes rating of the motor (times the service factor). However, a motor loaded below 60% of the full load amperes is less efficient and the power factor will also decrease, so it is all about finding the right balance.
An imbalance between currents can signal a problem with the motor windings e.g. different resistances on field windings caused by internal shorts; imbalances should generally be under 10%.
Current Imbalance = Highest Deviation from Average ÷ Average (of the Three Phase Readings)
In extreme cases, often caused by an open fuse, current imbalance can lead to single phasing where there is no current whatsoever on one of the three phases.
Variable speed drives do not have inrush current, but motors started by mechanical starters do – this inrush current is approximately 500% on older motors and can even be up to 1,200% on energy efficient ones. However, if it too high it can lead to both voltage sags and nuisance tripping.
The Fluke 374 clamp meter has the unique function of being triggered by the inrush current and capturing its true value.
Certain motors are subject to shock loads (peak loading) which can cause a high enough current surge that the overload circuit in the motor controller is tripped. A clamp meter’s min/max function can be used to record the worst-case current drawn by shock loads.
Using Clamp Meters Safely
Only trained, experienced electricians with a good knowledge of both electrical systems and the equipment being tested should ever perform testing or modification of electrical systems; the voltage and currents in electrical power systems can cause serious injury and even death from electrocution and burns.
We cannot anticipate every possible precaution that must be taken when performing these measurements, however as the absolute minimum you should:
- Use appropriate safety equipment (e.g. safety glasses, insulated gloves, insulating mats etc.)
- Be certain that, in any situation where you will be in direct contact with circuit components, all power has been turned off, locked out and tagged, and can only be turned back on by yourself
- Read and understand all applicable manuals before starting work, paying particular attention to all safety precautions and warnings
- Only use instruments such as clamp meters for the purpose for which they were intended; other than being dangerous, by using equipment in a manner not specified by the manufacturer you may also void the warranty.