Application Note # T43

Technical -- Locating Wiring in Walls

Introduction
(1) Cables in metallic (copper or steel) conduit:
(2) Screened cables (usually computer or data-comms cabling):
(3) Power cables in non-metallic ducts or plaster:
(4) Unscreened signal (non-power) wiring:

Introduction

The Protovale Pacetrace Live Cable Locator is available for the location of buried current-carrying power (50Hz) cables.
As with all live cable locators, it will not detect cables which are not live; nor will it detect cables which, though live, are not carrying any current (that is, are not loaded).
However it is not intended , and therefore not suitable, for locating low-current wiring inside buildings or internal walls.

The location of wiring in walls poses a certain number of problems:

(a) a cable may be live;
(b) a cable may be live but not loaded;
(c) a cable may be isolated (dead);
(d) there may be a need to locate non-power cables, eg computer or telephone wiring;
(e) in nearly all cases, the actual mass of metallic copper is small.

The only saving grace is that the maximum detection range required is normally quite shallow, often only about 30mm or less.

This Note is concerned with the location of dead (non-current-carrying) cables.

If a power cable does happen to be carrying a current, the detection sensitivities quoted below will not reduce. The signal indicated by the instrument may become 'modulated' by 50Hz radiation, with the audio output changing to a raucous growling sound and the meter needle vibrating; if this is observed, it will indicate that the cable is live, but lack of such an effect does not guarantee that the cable is dead. If cables are in steel conduit, any 50Hz radiation will be significantly attenuated.
In general, data-comms cables do not radiate as much signal (and indeed, the new EMC regulations stipulate that they must not), so no such AC signal pick-up should be expected.

Four types of situation are considered here:

(1) Cables in metallic (copper or steel) conduit:

The Imp Rebar Locator and the Rebar Plus are both suitable (pipes and conduit appear as hollow bars to these instruments).

Steel conduit is very easy to locate, at depths of up to at least 80mm; and the direction of run is very easily established. If the Rebar Plus is used, it is also possible to measure the approximate depth of the conduit beneath the surface of the wall.
Copper pipes give a similar signal which, though weaker, is still adequately detectable (up to depths of at least half that of steel conduit); and the direction of run can also be determined although not quite as sharply.
The only difficulty is that it will not be easy (and therefore not guaranteed possible) to distinguish between cable conduits and water- or gas-pipes.

If an Imp Stainless-Steel Wall-Tie Locator is available, it may also be tried:- in both cases, the 6S head should be used; for steel conduit, the electronics should be switched to 'A' or 'N' (not 'S'), and the ZERO knob will need to be turned back to reduce sensitivity; for copper, the 'S' position should similarly not be used unless it is necessary to increase sensitivity.
Note that all the above refers specifically to metallic conduit, and excludes plastic conduit or ducts.

(2) Screened cables (usually computer or data-comms cabling):

Up to a point, the outer screening of these cables behaves somewhat like conduit as in (1) above; however both the diameter and thickness are usually considerably smaller, and the screening is either in the form of a thin foil or else woven braid, so there is normally not enough suitable signal for either of the rebar locators (except perhaps in the rather special and unlikely case of armoured power cables).

The Imp Stainless-Steel Wall-Tie Locator , using its 6S head and switched to S mode, will often be suitable; and some typical results are shown below:

Data cable type Detection range

4-core, braid-screened 0 -- 10 mm

4-pair, foil-shielded 10 -- 20 mm

URM76 coaxial 20 -- 25 mm

12-core, braid-screened 30 -- 35 mm

Data cable type		Detection range
4-core, braid-screened		0 -- 10 mm
4-pair, foil-shielded		10 -- 20 mm
URM76 coaxial		20 -- 25 mm
12-core, braid-screened		30 -- 35 mm

Note that the shallower distance under "detection range" yields a clear signal, and the deeper distance corresponds to a weaker signal which an inexperienced operator may or may not recognise.
The results above were obtained using unterminated cables; termination of a cable, or connection to electronic equipment, may affect these figures and may even increase the signal.

(3) Power cables in non-metallic ducts or plaster:

With no screening/armouring/conduit, the rebar locators are not suitable.

The Imp Stainless-Steel Wall-Tie Locator (as in (2) above) will detect twin-and-ECC cables (BS6004 table 5, type 6242Y) as follows:

Copper cross-section Detection range

1mm² 35 -- 45 mm

1·5mm² 45 -- 55 mm

2·5mm² 50 --60 mm

4 -- 6mm² 50 -- 60 mm

Copper cross-section		Detection range
1mm²		35 -- 45 mm
1·5mm²		45 -- 55 mm
2·5mm²		50 --60 mm
4 -- 6mm²		50 -- 60 mm

Note that the shallower distance under "detection range" yields a clear signal, and the deeper distance corresponds to a weaker signal which an inexperienced operator may or may not recognise.
In the table above, the smaller three cable sizes measured had solid conductors, and the largest two had stranded conductors.

(4) Unscreened signal (non-power) wiring:

(eg telephone wiring) :

This situation provides the least amount of detectable metal of the four cases considered.
The Imp Stainless-Steel Wall-Tie Locator is the only hope-- and even then, expectations are not high.
Solid-cored conductors may be detectable at up to a few millimetres; but it is wisest to assume that stranded flexible conductors will not be detected.

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