Grade selection
Black, red or yellow mastic, and which joint takes which
Put the wrong grade at a screen cut and nothing goes wrong on the day.
The mistake that surfaces years after the trench is closed
The three mastics behave the same way in your hands. They press flat, they hold, they vanish under the wrap. Colour is the only thing separating them, and the colour goes out of sight the moment the sleeve recovers.
Put black at a screen cut and the wrap still closes. The void is filled. Water is kept out. Nothing on site tells you anything is wrong.
The published figures tell you. Black mastic is specified at a dielectric constant of 2 to 3. Yellow is specified at 10 minimum. The method is ASTM D150.
That number is the whole reason the yellow grade exists. Cut a screen on a medium-voltage cable and the electric field crowds at the cut edge. A filler with high permittivity spreads that field along the cut. A filler with low permittivity leaves it where it was.
The manufacturer's own wording for yellow is stress relief void filling mastic, used on connectors and at screen cuts. Read it as two jobs carried by one strip. It fills, and it grades. Black does one of the two.
Red answers a third question. It goes on medium-voltage termination breakout boots, non-tracking tube and lugs, where a seal has to stand off voltage and be stretched around a shape. It is the only grade on the sheet specified above 0.1 MPa tensile.
Three grades, three questions
Every row except the last is read off the manufacturer's Mastic Tape datasheet. The last row is this reference's reading of that sheet, not a published prohibition.
| Decision | Black HBMT | Red HRMT | Yellow HSCM |
|---|---|---|---|
| The stated job | Sealing and waterproofing LV and MV joints, terminations and telecom installations | A watertight seal on MV termination breakout boots, non-tracking tube and lugs | Stress relief and void filling at connectors and screen cuts on MV joints and terminations |
| The property it is bought for | Adhesion and water sealing | Volume resistivity, 1 × 10¹⁴ Ω·cm min. | Dielectric constant, 10 min. |
| Codes on the selection chart | Nine, HBMT-01 to HBMT-15 | Five, HRMT-01 to HRMT-19 | Two, HSCM-06 and HSCM-09 |
| Thickness supplied | 2.5 mm | 2.0 mm and 1.7 mm | 1.7 mm |
| Width supplied, minimum | 35 mm | 35 mm | 20 mm and 25 mm |
| Longest length charted | 25000 mm, HBMT-14, on a roll | 25000 mm, HRMT-05, on a roll | 450 mm, HSCM-09, in strips |
| Supplied on a roll | Yes, four codes | Yes, one code | No, strips only |
| Where this is the wrong part | A stress-relief position at a screen cut | A deep void that has to be filled and held | A general water seal on a jacket or a gland |
The three grades on published figures
| Property | Method | Black | Red | Yellow |
|---|---|---|---|---|
| Tensile strength, MPa | ASTM D638 | 0.1 min. | 0.5 min. | 0.1 min. |
| Ultimate elongation, % | ASTM D638 | 300 min. | 300 min. | 300 min. |
| Density, g/cm³ | ASTM D792 | 1.30 ± 0.2 | 1.20 ± 0.2 | 1.30 ± 0.2 |
| Service temperature, °C | IEC 216 | −20 to +90 | −20 to +90 | −20 to +90 |
| Dielectric strength, kV/mm | ASTM D149 | 6 min. | 10 min. | 15 min. |
| Volume resistivity, Ω·cm | ASTM D257 | 1 × 10¹⁰ min. | 1 × 10¹⁴ min. | 1 × 10⁹ min. |
| Dielectric constant | ASTM D150 | 2 to 3 | 3 max. | 10 min. |
| 50 Hz loss factor | Not stated | 0.035 | 0.035 | 0.035 |
| Adhesion and self-amalgamation | Not stated | Good | Good | Good |
Typical values from the manufacturer's Mastic Tape datasheet. Two rows carry no method: the 50 Hz loss factor is 0.035 for all three and separates nothing, and adhesion is rated in words rather than in newtons.
Not published: how to name a grade once the liner is gone
The code prefix is the identification, not the colour. HBMT is black, HRMT is red, HSCM is yellow. Beyond naming those three colours the datasheet publishes no colour reference, no shade tolerance, no marking carried on the mastic itself and no shelf life. Under a work lamp at dusk a dark red strip and a black one are not reliably different. Keep strips in marked packaging until the moment they go on, and write the code onto the joint record rather than the colour. If a loose strip cannot be named, ask the manufacturer instead of reading it off the shade.
What each substitution costs you
Five swaps that happen on site. The consequences are this reference's reading of the published figures, not a warning list the manufacturer prints.
- Black where yellow belongs. The connector shoulder and the screen cut get filled and sealed. The field is not graded. Black is specified at a dielectric constant of 2 to 3 against yellow's 10 minimum, so the stress stays concentrated where the cut left it. This is the swap that charges you years later rather than on the day.
- Yellow where black belongs. You have moved the lowest-resistivity grade on the sheet into a general water seal. Yellow is 1 × 10⁹ Ω·cm minimum against black at 1 × 10¹⁰ and red at 1 × 10¹⁴. It will seal. It also puts a deliberately less resistive layer where nothing asked for one.
- Black where red belongs. Boot legs, non-tracking tube and lug seals are red's listed job. Black holds 6 kV/mm against red's 10, and 0.1 MPa tensile against red's 0.5. The tensile row is the one that bites. Red is the grade specified to be pulled around a leg and held there.
- Red where yellow belongs. Red is the highest-resistivity grade on the sheet and the thinnest charted, at 1.7 mm. Neither property helps at a screen cut. A stress-relief position wants permittivity and bulk, and red is specified for neither.
- Any grade as an insulation build-up. Butyl mastic does not cure and keeps moving under load. Wrapping a wall of it produces a soft body that creeps out of an unrestrained gap. Building insulation by hand is the non-shrinkable high voltage tape's job, not a mastic's.
Where to go from here
- The full HBMT, HRMT and HSCM selection chartSeventeen rows: width, length, thickness and whether the code arrives as strips or as a roll.
- What mastic tape is, and what it actually doesCold flow, void filling and the waterproof question, without the grade comparison.
- Non-shrinkable high voltage insulation tapeThe part that builds an insulation wall where no mastic should be asked to.
- Every method named on this pageASTM D149, D150, D257, D638 and D792, plus IEC 216, read without taking them on trust.
- Heat shrink tape against electrical tapeThe other tape decision on this site, argued on a different axis.
Order the code, not the colour
Send the joint or termination kit reference, the cable size, and the position the mastic goes into: connector, screen cut, boot leg or gland. What comes back is a code.