TOMORROW'S TECHNOLOGY TODAY
GRUMBLER CABLE IDENTIFIER
Grumbler identifies LV cables by drawing a unique very low frequency signal through them and seeking it along the route with a directly applied inductive search coil connected to a tuned receiver. These signals do not stray into adjacent cables. The system was developed to distinguish LV cables from HV, it is also used to identify LV cables and the phase to which a consumer is connected.
Intending users are advised to study these instructions in depth, to fully understand them and to make the described experiments indoors with a specially prepared length of steel tape armored cable before attempting work on live systems.
Operating experience has built up confidence that, if the user follows every step of these instructions, the receiver will not give a continuous response on a cable live at HV. Nevertheless, to avoid human error, precautions must be taken as described in the statement overleaf before exposing the insulation of conductors.
For the majority of situations it will be necessary to prevent signal current sharing neutrals, sheaths and earth paths by operating the Grumbler Modulator at 415 volts.
In simple applications such as finding the LV main which supplies a particular consumer or lighting column, 240 volt working may be accurate enough.
The infinite permutations of cables in supply networks make it impossible to prove and claim that under all circumstances the Grumbler will give clear, unambiguous indication on the required LV cable. Nevertheless it has been shown to be a very useful instrument, easing difficult decisions, saving time and money for distribution companies and contractors.
It is intended to fail safe by either not responding on any cable or by indicating on more than one LV.
The receiver will respond, briefly when it is first switched on and when the search coil is moved either quickly near to ferrous materials or very quickly through the air.
This is normal behavior and reassures the operator that the equipment is functioning.
DURING TESTING ALL MOVEMENTS OF THE SEARCH COIL MUST BE SLOW and DELIBERATE WHILST THE OPERATOR SEEKS A SUSTAINED RESPONSE.
DESCRIPTION OF EQUIPMENT
The device switching the pulses is built in a rugged glass/resin moulded case of similar pattern to those used for portable transformers, distinctively colored yellow. lt is of double insulated construction for earth free operation.
Connection is by two rubber sheathed cables each terminated at the free end in a moulding containing a 7 amp fuse. From each fuse holder projects a 4mm plug pin shrouded by a rubber sleeve. The pins can be fitted either with insulated crocodile clips for use in distribution pillars link boxes etc. or into a single phase connector for 13 amp sockets.
The 7 amp fuses are rated to clear 10,000 amps at 440volts.
A 3 amp.,440v. fuse within the case gives further protection. The single phase connector is fitted with a 13 amp fuse.
A fraction of the A.C. input is rectified and regulated to power an oscillator which triggers a pair of triacs, switching a burst of positive current pulses followed by a burst of negative through a series/parallel array of low loss inductors at a rate of eight periods per second.
Pulses are automatically regulated by control of firing delay to be of approximately 5 amps peak, 1.0 amps r.m.s. max. for input ranges of 390-440v and 220-255 volts at 50Hz.
A light emitting diode (LED) flashes at signal frequency to confirm the unit is operating. For safety in case of damage the LED is isolated from power circuits by a current transformer.
Power dissipated is a maximum of 25 watts enabling operation for up to two hours in the unventilated, drip proof case.
The inductors act as a heat sink, slowing temperature change.
In EXPORT versions a self-resetting thermal switch disconnects triac drive pulses if the Modulator is left energized for extended periods. When the switch has operated the LED changes from flashing to a dull glow. Cooling is slow, reset time can be 20-40 minutes.
Semiconductors and capacitors are suitably rated for systems
up to 415v + 6% but this is the absolute limit. Higher voltages can cause rapid heating, internal fuse and component failure.
SEARCH COIL A rectangular coil sealed in a heat shrink sleeve, then formed to 60mm radius and flexible enough to fit the surface of any cable when pressed firmly into contact by an operator. Magnetic fields concentric with a cable induce equal, opposing voltages in the coil sides. This prevents response to cable sheath currents. The flexible lead is terminated in a t jack plug for connection to the receiver.
The receiver is built in an ABS plastic case and protected by a leather sleeve with a strap allowing it to be worn round the operator's neck. An aluminum screen between sleeve and receiver reduces radio frequency interference.
The exposed top face has a rotary switch with three positions
'OFF', 'STA', and 'OTHER'
'STA' ~ives full sensitivity for steel tape armored cables. 'OTHER position for wire armored or unarmored cables reduces sensitivity by a factor of 10. (see operating procedure).
There are also an LED (yellow) to indicate battery condition, an LED (red) to flash when signals of the correct form and in excess of the internally preset threshold are induced into the search coil. A ~inch jack socket accepts search coil signals.
Within the case but clearly audible is a bleeper which sounds in response to received signals.
Two drawers in the bottom of the case each house a PP3 sized 9 volt alkaline primary battery.
Internally there is a screened, resin potted receiver module to amplify the search coil signal, reject 50Hz and other unwanted noise, select and measure the 8Hz component.
Valid signals are passed to an open printed circuit board which drives the bleeper and the LED’s.
When switched to 'STA' or 'OTHER' the battery condition LED (yellow) lights and remains lit.
The flashing LED (red) and the bleep respond for about six seconds, then stop. After this the receiver remains silent until the search coil is applied to a cable carrying modulator pulses in excess of the threshold or is moved rapidly. On the 'STA' setting the search coil has to be within 50mm of an unarmored cable and in immediate contact with the serving over steel tape amour before the receiver can respond.
CABLE IDENTIFICATION PROCEDURE
Whatever signal is impressed for cable identification; there is always the possibility of a nearby consumer operating electronically switched apparatus which demands a similar pattern of current pulses.
The operating procedure detailed on the next page guards against this by making negative tests before and after the modulator is used.
DO NOT TRANSMIT ON PERSONAL OR MOBILE RADIOS AT THE TEST SITE DURING CABLE SCANNING and REMEMBER THAT PASSERS BY MAY BE DOING THE SAME THING
OPERATING PROCEDURES ALL STEPS TO BE TAKEN
1. Find a convenient point for connection of the modulator PREFERABLY PHASE to PHASE or, if this is not possible, phase to neutral so that it will draw current through the wanted cable at the selected test site. Do not connect it at this time.
2. Attend the test site with receiver and search coil.
Test sites within substations MUST be at least 2 meters from power transformers to avid direct induction to the search coil
3. Clean off the surface of all exposed cables for their
full circumference over at least lS0mm of length, down to the outer serving or, if the serving has rotted, the amour.
4. Plug search coil into the receiver.
5. Switch the receiver to 'STA'. Check that the yellow 'BATT' LED lights and stays on. (If it does not, change both batteries). Check that bleep and flashing LED operate for about 6-10 sees.
FIRST NEGATIVE TESTS
6. Apply the search coil to each cable in turn, held so that the signal cable is tangential to the power cable. Press the coil with the palm of a hand to conform with the surface. Very slowly scan the full circumference of each cable.
7. If signals are not received from any cable there is no interference from consumer's equipment or radios at this time.
POSITIVE TESTS see Fig 2 overleaf.
8. Connect the modulator between two phases or phase to neutral at the point selected in Step 1. There is usually a small flash so WEAR RUBBER GLOVES and attach the clips positively, each with a single movement, to avoid transient stresses which can blow the internal fuse. If the modulator is in the open, guard it and post 'Danger' notices to avoid interference by the curious.
9. Confirm that the modulator LED is flashing, if not, check that the source is live, that connections are clipped to bare metal and that fuses and test leads are in good order.
10. Return to test site, switch receiver to 'STA', scan the full circumference or the conductor lay length of each cable very slowly with the coil pressed into contact. Note any which now give a sustained signal.
Signal strength from all except single core cables will vary as conductor lay is followed. Weak signals may vary cyclically when the modulator oscillator beats with system frequency
Clearance of at least 80mm is necessary from and between unarmored and wire armored cables in the test area
OPERATING PROCEDURE POSITIVE TESTS Continued. 'OTHER' CABLES
11. If any of the cables which gave a response are not
steel tape armored, scan them again with the receiver switch selected to 'OTHER'. If they still respond it confirms, with an enhanced safety margin, that they are LV cables carrying the modulator signal. (See SAFETY MARGINS on page 11).
12. Steel tape armored cables which responded only when the modulator was operating are of the same LV network.
Other types of cable which responded on the 'OTHER' setting with the modulator connected are also confirmed to be LV.
REPEAT NEGATIVE TESTS
13. Disconnect the modulator and repeat the negative tests described in para.G. If there are again no interfering signals there is a very strong possibility that there were none during the positive tests.
If there is interference the positive test results are suspects.
MULTIPATH SIGNAL CURRENT
14. If, with phase to neutral operation, there is a response from more than one cable, it indicates that the return signal current is shared by several neutral conductors and earth paths bonded at remote points. Where conditions permit, the temporary opening of a neutral link may help. If this is not possible, resite the modulator to allow phase to phase connections.
15. If, in a ring or parallel feeder LV system of s.t.a. cables it is not possible to get a response from those carrying a phase to phase signal, the cause could be current sharing.
The difficulty may be overcome if the modulator connections can be changed to maximize field strength by using a diametric pair of conductors.
Alternatively, if load conditions allow, a temporary break of parallel would be helpful.
IDENTIFICATION OF DEAD CABLES
15. The Grumbler can also be used to identify dead HV or LV cables if a temporary supply of 240 or 415v. 50Hz. 3 amps is applied to the cable at one end with the modulator in series and two cores looped at the remote end.(Fig.3).The modulator LV supply cable should be identified beforehand to avoid confusion.
Modulator current pulses are present, scaled down, in the HV cable to the distribution transformer primary.
An in-built safety margin must be provided to ensure the receiver cannot respond to these. A receiver set to barely respond to signals from the smallest LV tape armored cable of the old 0.0225 sq. inch size will detect signals at 7% of this level from unarmored HV or LV cables.
Fortunately several factors reduce signal strength on the HV side these are:-
Ratio of Transformation Insulation
HV Line Waveform
LV System Capacitance
RATIO of TRANSFORMATION: Primary current is reduced by the inverse of the transformer ratio.
With modulator connected phase-neutral, 28 : 1 for 6.6 kV and 46 : 1 for 11 kV.
With modulator connected phase-phase, 16 : 1 for 6.6 kV and 26 : 1 for 11 kV.
Thicker insu1ation,and the belting papers and tapes of HV cables increase the radial distance from conductors to search coil, reducing signal strength! by some 25%
H.V. LINE WAVEFORM:
Modulator current has a significant third harmonic content which appears in the delta connected transformer primary but not in the HV supply cable,
reducing signal strength by a further 25%.
L.V. SYSTEM CAPACITANCE
The modulator draws a lagging current from the system. Some of this may be supplied by the distributed capacitance of the LV cable network, further reducing signal current in the transformer HV cable.
When all these effects are taken into account, the safety factor is 2/1 at normal sensitivity ( set to 'STA') for the very worst case of a modulator connected phase to phase and the transformer fed at 6.6 kV by an unarmored cable.
With the modulator connected phase-neutral to a transformer fed at 11 kV by an unarmored cable it is 5.5/1
Tape armored cable factors range from 28 : 1 to 77: 1.
The safety factor will be multiplied by 10:1 for steel wire, aluminum armored and plain sheathed cables by making a final check with the receiver selected to 'OTHER'.
BEST ADVICE FOR GRUMBLER
1 WHENEVER POSSIBLE CONNECT THE MODULATOR PHASE to PHASE
2 ALWAYS CONNECT PHASE to PHASE WHEN DISTINGUISHING LV from HV CABLES
2 FOR STRONGEST SIGNAL CONNECT THE MODULATOR TO DIAMETRICALLY OPPOSITE CORES
3 ALWAYS MOVE THE SENSOR COIL VERY SLOWLY ROUND OR ALONG THE CABLE UNDER TEST
4 LOOK FOR SIGNAL AMPLITUDE CHANGES AS THE CIRCUMFERENCE OR LENGTH IS SCANNED, THIS CONFIRMS THE SIGNAL IS FROM CONDUCTORS AND NOT THE SHEATH
5 IGNORE ALL SIGNALS GENERATED BY RAPID MOVEMENT OF THE SENSOR COIL
6 USE THE LOW SENSITIVITY "OTHER" RANGE SETTING FOR ALL CABLES OTHER THAN STEEL TAPE ARMOUR ED
7 DO NOT TRANSMIT ON PERSONAL OR MOBILE RADIOS AT ** THE SITE DURING CABLE SCANNING ** LOOK OUT FOR OTHER PERSONS USING THE SAME
8 DO NOT REMOVE THE ALUMINIUM SLEEVE BETWEEN RECEIVER AND LEATHER CASE. IT IS THERE TO MINIMISE INTERFERENCE FROM MOBILE and PERSONAL TELEPHONES
OPTIONAL ACCESSORIES TO EXTEND "GRUMBLER" RANGE
Extra large sensor for STA cables over O.25sq inch.
Small probe sensor for exploring distribution boards to identify the phase feeding a particular consumer.
Further confirmation can be made by moving one modulator lead to a different phase . if the point of maximum signal moves along the lay it must be the required cable
LIMITATIONS OF USE
LV SYSTEM VOLTAGE.
Modulators are designed for 415/240 volt 50Hz. systems with normal voltage fluctuations of +/- 6%.
They MUST NOT be used on LV supplies in excess of 440 volts. This does NOT mean they can be used on systems with a declared nominal voltage of 440.( which could reach 470v ). Nor are they to be connected between lines on two phase systems of 480/240 volts ( which could reach 510v )
Engineers who wish to bench test a modulator from a variable voltage supply must avoid the range 265-375 volts over which the auto ranging control may not keep line current in check and the internal 3 amp. fuse could blow.
HV SYSTEM VOLTAGE.
Grumbler equipment should not be used on unarmored HV cables associated with transformers operating on primary voltages of less than 6.6 kV.
Do not allow the modulator to overheat. This may happen after two hours operation in UK summer.
Only one Grumbler modulator to be connected to a particular LV system at one time.
Modulator and Receiver are matched, optimized and supplied in pairs marked with their Serial Numbers. They must only be used in pairs as supplied and, preferably, with their own search coil.
A preset resistor on the p.c.b matches the modulator to the receiver. This setting must not be altered. The receiver has built in facilities for the connection of external test gear to aid tuning but the work can only be done satisfactorily by our staff using workshop facilities. Detuning lowers sensitivity.
To prevent misleading responses the search coil must not be used within 2 metres of a power transformer or 3 metres of the modulator and away from mobile/portable phones.
GUARANTEE & REPAIRS
The specialized form of this equipment and the need for accurate calibration make it imperative that all repairs are carried out by GFF Ltd. Any failures caused by component faults or manufacturing defects will be repaired free of charge by GFF Ltd. for twelve months from date of delivery. This does not cover:- use on voltages in excess of 440 and/or supplies at other than 50Hz.,failed fuses, damaged leads, physical damage, internal interference, damage caused by water or leaking batteries, overheating by extended energization •• We reserve the right to charge for return delivery.
After the 12 months GFF Ltd will continue to provide an economical, rapid repair service.
FLOAT POWER SYSTEMS & CONTROLS © 2010