What Is a Wall Scanner? Types and Selection Criteria
A wall scanner is a handheld detector that finds metal pipe, wooden joists and live cable behind a wall or ceiling and signals their location with light and sound.
A wall scanner is a handheld detector that is moved across a wall or ceiling surface to detect metal pipe and rebar, wooden joists and live cable behind it and to signal their location with light and sound. It is used before drilling, chasing and anchor installation to avoid hitting a pipe, rebar or live cable and to find a joist or stud in a partition wall.
Types
- Multi-material scanner: Detects metal, wood and live AC cable in separate modes. UNI-T UT387E, UT387S and UT387LM in the catalog are in this group.
- Voltage and metal detector: Mainly looks for metal and live cable. UNI-T UT387D is of this type.
- Multi-function digital detector: Combines several detection functions in one device. Schneider Electric IMT23104 is a 4-in-1 digital detector; check its functions on the product page.
Two Unit products that are not wall scanners are also listed in the category: the UT661D pipe blockage detector and the UT665 borescope snake camera. They do not scan behind walls; they are for locating a blockage or looking inside narrow cavities.
Detection depths
The product data of the UT387 series gives the following maximum depths by material:
| Material | Maximum depth in product data |
|---|---|
| Ferrous metal (pipe, rebar) | 80 mm, 100 mm |
| Non-ferrous metal (copper, aluminium) | 80 mm |
| Wood | 20 mm, 38 mm |
| Live AC cable | 40 mm, 50 mm |
Compare the value per model on the product page.
Selection criteria
- Material to find: If you only look for cable and pipe, a model that detects metal and voltage is enough; for joists or a timber frame behind plasterboard you need a wood mode.
- Depth per material: The plaster thickness and the depth of the object must stay below the depth the device gives for that material.
- Alert type: Models with a display show the centre and approximate depth of the object more easily; models with only light and sound alerts locate the object.
- Power: The catalog has 9 V battery and rechargeable models; for frequent use a rechargeable model saves on batteries.
Common mistakes
- Ignoring de-energised cable: Voltage detection works only on live AC cable. A cable whose circuit breaker is off, or one carrying DC, does not show in this mode; also scan for the copper conductor in metal mode.
- Relying on the scanner instead of isolating: Switch off the circuit breaker of the circuit concerned before drilling or chasing; the scanner is only a pre-check.
- Calibrating in the wrong place: If the device is not placed on the surface and calibrated at a point with nothing behind it before sliding, the result drifts.
- Trusting the result on damp or foil-backed surfaces: Moisture, metal-foil insulation and dense rebar reduce the detection depth or cause false alerts.
- Scanning in one direction only: To pin down an object, scan the surface both horizontally and vertically.
Brands in the catalog
The Wall Scanners category has 7 active products: Unit 6 (UT387E, UT387S, UT387LM, UT387D, UT661D, UT665), Schneider Electric 1 (IMT23104). Five of these are wall scanners or detectors. To trace a cable with a transmitter, see Cable Testers and Tracers; to confirm absence of voltage before work, see Voltage and Continuity Testers. Industrial borescopes are in the main Electrical and Electronic Testers category.
Frequently asked questions
What is the difference between a wall scanner and a cable tracer?
A wall scanner detects only metal, wood or live cable directly behind the surface it scans. A cable tracer follows the signal of a transmitter connected to the cable and finds the cable's route and which circuit it belongs to.
Does a wall scanner find de-energised cable?
Not in voltage mode. The copper conductor of a de-energised cable may be detected as non-ferrous metal, but this depends on depth and cable size.
Why is the stated detection depth not reached?
The stated values are maximum values. The wall material and its moisture, the size of the object and whether it lies square to the surface reduce the actual depth.
How do you find a stud or joist in a plasterboard wall?
Metal studs are found in metal mode and a timber frame in wood mode. Mark both edges of the stud and drill in the middle.