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How to measure a shiny busbar with a strip of electrical tape

A strip of matt tape converts a mirror into a surface a thermal camera can read, which fixes a problem no camera setting solves on its own. The method is simple and the sequencing is not, because the tape has to go on while the panel is dead and the measurement has to happen while it is live.

By the Akermin Editorial Team · 8 min read

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Stick a strip of matt electrical tape on the bar while the panel is isolated and dead, then measure the tape rather than the metal once the panel is back in service. Matt PVC tape has an emissivity conventionally taken as about 0.95, so it converts a mirror finish into a surface that radiates almost everything the camera needs, and the reading it gives is the bar temperature because the tape is in intimate contact with a large thermal mass. The critical constraint is sequencing: the tape has to be applied with the circuit proved dead, which means the whole technique belongs to a planned shutdown, and the tape then stays in place as a permanent target for every survey afterwards.

Why a strip of matt tape fixes a reading that no setting can

A bare conductor with a bright finish emits very little and reflects almost everything, so most of what the camera collects from it came from the room. Entering the correct low emissivity into the camera helps, but it helps less than people expect, because dividing out a small emitted fraction amplifies every other uncertainty in the measurement. A ten percent error in your assumed emissivity of 0.05 is a large error in the final temperature, and you also need the temperature of the surroundings being reflected, which is rarely a single value inside a switchroom.

Tape sidesteps the whole problem by changing the surface instead of the arithmetic. With emissivity around 0.95, the emitted term is nineteen times the reflected term, the reflected contribution becomes a rounding error, and the measurement stops depending on what the bar is reflecting at all. The question changes from what is this mirror showing me to what temperature is this piece of tape, and the second question has a straightforward answer.

There is a second benefit that matters more over time. A taped patch is a repeatable target. Bare metal oxidises, gets handled, gets cleaned, and its emissivity drifts accordingly, so a reading taken this year is not comparable with one taken from the same spot before. A tape patch left in place fixes the measurement geometry and the surface property together, which turns a one off number into a trend.

The step that has to happen with the panel dead

This technique has a hard prerequisite that no amount of care substitutes for. Applying tape means putting a hand inside the enclosure and touching a conductor, so it happens only with the circuit isolated, locked out, tagged and proved dead at the point of work, by someone competent to do that and following whatever safe system of work governs the installation.

Proving dead is a test, not an assumption. A non contact detector tells you where voltage probably is, and that is useful for the survey, but establishing absence of voltage is done with a tester rated for the installation and verified against a known source before and after. The roundup of electrical testers covers the instruments built for that step, and the methods are in the guides to measuring AC voltage with a multimeter and measuring resistance safely. The resistance check is worth doing while the panel is open anyway, because a joint you intend to monitor thermally is a joint you can characterise directly while it is dead.

The practical consequence of the dead panel rule is that the thermal survey and the tape application are different visits. Plan it that way: tape every joint you expect to monitor during the shutdown, photograph where the patches are, then survey under load afterwards. Trying to compress the two into one visit is what leads people to work on live equipment they should not be touching.

Getting the tape to the same temperature as the copper

The tape reports its own temperature, so the method only works while the tape and the bar are at the same temperature. On a substantial busbar that is essentially automatic, for three reasons: the tape is thin, its thermal mass is negligible beside the bar, and the bar conducts heat along itself far faster than a small patch of tape can impede it. Allow a minute or two after the load stabilises and the patch has arrived.

The cases where it fails are worth naming.

  • Thin conductors and small components. On a small terminal or a fine wire, the tape can cover a significant share of the surface area that part uses to shed heat, and it then behaves as insulation. The patch reads high and the component runs hotter than it otherwise would.
  • Poor contact. A patch that is bridging a curved or dirty surface traps an air gap, and air is a good insulator. Burnish the spot, press the tape down fully, and avoid wrapping over an edge.
  • Air movement. A forced air stream across a taped patch cools the tape more effectively than it cools the bar. Where a panel has fans, note which patches sit in the airflow.
  • Fast transients. Immediately after a large load step the bar changes faster than the patch follows. Let it settle.

Size the patch to the camera as well. The patch needs to fill a clear block of pixels at the distance you will be standing, which for most handheld cameras at a few metres means a target a few centimetres across rather than a narrow stripe. If the camera does not publish a field of view, the empirical check is to look at the live image and confirm the patch occupies an unambiguous area of pixels rather than a single sampled point.

Three emissivity free ways to get the same answer

Tape is one route to a trustworthy number. Three others remove the emissivity question entirely, and each has a different limitation.

Route What it needs What it still depends on Where it fails
Matt tape target A planned shutdown to apply it Tape and bar being at the same temperature Thin conductors, poor contact, forced airflow
Contact thermocouple Physical access to the surface Good contact and a settled probe Cannot touch live conductors; one point only
Phase to phase comparison Identical conductors with identical finishes Equal loading across the phases Gives a relative answer, not an absolute temperature
Measure the insulation or boot Nothing; works on the first visit Thermal contact between covering and conductor Underreads, by an amount that is not calculable

The contact route is worth taking seriously because it is exact. A thermocouple reaches the temperature of what it touches and reports it with no surface assumption at all. The BTMETER 570T-APP clamp multimeter ships with a thermocouple probe covering minus 4 to 1832 degrees Fahrenheit alongside its 400 and 1000 amp current ranges, 45 millimetre jaw, 4000 count display and data logging, and it carries CAT IV 600 V and CAT III 1000 V ratings. That combination is useful because the same instrument gives you the load and the temperature, and load is the context a temperature reading needs.

The comparison route is the one most people underuse. If three bars in a panel have the same finish, the same geometry and the same expected current, then whichever is warmest is the anomaly, and that conclusion holds even when every absolute number on screen is forty degrees low. Emissivity error is systematic, so it cancels in a comparison between identical surfaces. The failure mode is unequal loading, which is why the current measurement matters.

What the temperature alone will not tell you about the fault

A warm joint and a cool joint are not automatically a good joint and a bad joint, because temperature rise depends on current as well as resistance. Power dissipated in a connection goes as the current squared multiplied by the resistance, so halving the load quarters the heating. A badly degraded joint on a lightly loaded circuit can look perfectly healthy, and a sound joint on a heavily loaded one can look alarming.

That makes current the companion measurement. The Fluke T6-1000 PRO is designed for exactly this situation: its open fork measures AC voltage, current and frequency without touching a live wire, it fits conductors up to AWG 4 slash 0 through a 17.8 millimetre jaw opening, and it shows voltage and current at the same time. Knowing that a cool looking bar is carrying a tenth of its rated current changes the interpretation of its temperature completely.

A cool phase that is carrying no current at all is a different fault again, and it is a common one. An open phase, a blown fuse or a reversed sequence after maintenance will show as a thermal asymmetry that has nothing to do with joint quality. The Ideal 61-521 three phase motor rotation tester states phase sequence identification, motor rotation direction confirmation and open phase detection through colour coded clips, which settles that question in a few seconds rather than by inference from an image. Where the asymmetry is in a circuit rather than a panel, the instruments covered in electrical fault finders are the ones built to trace it.

A tape routine you can repeat at the next shutdown

Written as a sequence, with the division between dead work and live work made explicit.

  • During the shutdown, with the circuit isolated, locked out and proved dead: burnish each spot you intend to monitor, apply a matt tape patch a few centimetres across, press it down fully, and avoid edges and any possible current path. Record the positions with a photograph.
  • Still dead: take a resistance measurement across each joint you have taped, so the thermal trend has a baseline that does not depend on load.
  • With the panel back in service and treated as live: let the load stabilise for several minutes, then set the camera emissivity to the tape value if the control exists, and image each patch square on from a distance where it fills a clear block of pixels.
  • At the same time: record the current in each conductor, and the ambient temperature inside the enclosure. A temperature without a current and an ambient is not comparable with anything.
  • Afterwards: compare patches against each other first, then against the previous survey of the same patch. Leave the tape in place so the next visit measures the same spot.

The underlying idea is worth keeping even where tape is impractical. A thermal camera is reliable when it is pointed at a surface whose emissivity you know, and unreliable when it is not. Everything above is a way of arranging for the first situation rather than arguing with the second.

Frequently Asked Questions

What emissivity should I set for electrical tape?

Matt PVC electrical tape is conventionally taken as about 0.95, which is the same figure used for most building materials and matt paints. Glossy tape is lower and less predictable. If the camera has no adjustable setting, it is very likely already assuming a value close to 0.95, which is one of the reasons the tape method works so well on instruments with no control at all.

Can I stick the tape on a live busbar?

No. Applying anything to an exposed conductor means working inside the enclosure at arm length from live parts, and that is how people are killed or burned by an arc flash. The tape goes on during a planned shutdown with the circuit isolated, locked out and proved dead. The measurement then happens later, with the panel energised and the enclosure treated as live.

Does the tape change the temperature of the joint?

On a substantial copper or aluminium bar, not measurably. A thin strip of tape has negligible thermal mass and negligible thermal resistance compared with the bar it is stuck to, and the bar conducts heat along itself far faster than the tape can insulate a small patch. On a thin conductor or a small component the picture reverses, because the tape can cover a meaningful fraction of the surface the part relies on to shed heat.

What if I cannot get into the panel at all?

Then measure what is accessible and accept what that gives you. Cable insulation, lug boots and painted enclosure surfaces all sit near a high emissivity and are often in reasonable thermal contact with the conductor, so they report a damped version of the real temperature. An infrared window fitted during a shutdown is the permanent answer where repeat surveys are planned.

Is paint better than tape?

Flat paint gives a more durable and more uniform target and is the usual choice for a permanently monitored asset, but it is a modification to the equipment rather than a reversible addition, so it needs agreement from whoever owns the installation. Tape is reversible and good enough for most work. Neither should be applied anywhere it could enter a current path or compromise clearance.

Does a thermocouple need an emissivity setting?

No, and that is its main advantage. A contact sensor comes to the temperature of whatever it touches and reports that directly, so surface finish is irrelevant. The cost is that it must touch, which on an electrical conductor restricts it to isolated equipment or to a permanently installed probe, and that it reports one point rather than an image.