Technical Guidelines | Section F: Installation and Maintenance
F4 ACTIVE PROTECTION
OF HIGH-VOLTAGE CIRCUITS
& GRAPHICS
The British Sign & Graphics Association has used its best endeavours to ensure that the information given in these Technical Guidelines is accurate but it cannot accept responsibility for loss occasioned to any person acting or refraining from action as a result of the material in these guidelines. If in any doubt specialist advice should be obtained.
The content of this guideline is copyright to the British Sign & Graphics Association and may not be re-produced without the consent of the Association’s Council.
The two main hazards associated with high-voltage signs are:a) Risk of electric shock; and b) Risk of fire.
To reduce the likelihood of either of these hazards occurring, signs should be protected by means of suitable insulation and enclosures and by maintaining the correct creepage distances and clearances between highvoltage conductors and earthed metal or parts which may be touched. Collectively, these measures are known as passive protection, and the requirements are covered in:
BSGA Technical Guideline F3: “High-Voltage Installations”.
Further protection may be added to a circuit to ensure that, for example, even if an electric arc to earth may occur, the presence of the arc is detected before it has a chance to start a fire and the mains supply to the faulty circuit is switched off. Devices to do this include circuits which measure current or voltage (as appropriate) and react by operating a relay to switch off the mains supply to a transformer or by disabling the oscillator of an invertor or convertor. Protection by means of such devices is known as active protection and is the subject of this
Technical Guideline.
Active protection ensures that problems associated with the breakdown of passive protection are detected and suitable action taken. As will be discussed below, it should not be considered a substitute for passive protection which should remain the first line of defence against electric shock and fire.
The three types of active protection discussed in this Guideline are: a) earth-leakage protection; b) open-circuit protection; and c) detection of electric arcs which are not to earth.
2.1 Earth-Leakage Protection
Electric arcs from high-voltage connections to earth are a frequent cause of fires in signs. The circuit may be switched off, or otherwise disabled by the use of an earth-leakage protective device. The characteristics of such devices, their performance and ways in which they may be connected are discussed in Sections 3 to 11 below.
2.2 Open-Circuit Protection
There is no doubt that a sign where the tubes are not alight but the high-voltage is still present is more of a hazard than one where the tubes are lit. The latter provides a strong indication that a dangerous voltage is present. The use of an open-circuit protective device contributes to the safety of people by ensuring that signs which are still live but where the tubes are not alight (for whatever reason) are switched off. The extra protection afforded by open-circuit protection should be considered, particularly for signs installed within arm’s reach.
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A typical case might be a small window sign having exposed tubing with the extent of the protective enclosures restricted to the high-voltage connections and the tubing close to the electrodes. This arrangement would normally be satisfactory but if the tube was broken it might be possible to touch a live electrode through the broken end of the tube. Under such circumstances, use of an open-circuit protective device which switched the sign off as soon as the tube was broken would improve safety. The details of open-circuit protection are discussed in Section 12 below.
Note: It has been wrongly suggested that an open circuit protective device will protect against fires in signs (such as all plastic signs) where an electric arc to earth is unlikely. If a connection in a high-voltage circuit should part and the resulting gap is no more than a few millimetres, an arc might occur between the two conductors which has nothing to do with a current to earth. If this arc is prolonged, and if there are flammable materials in the vicinity, it is possible that these might catch fire.
Even if this does happen, the secondary current continues to flow and there is no easy way to detect the condition (see 2.3 below). Open-circuit protection has been advocated as a protection against such fires but for it to work one must assume that the two ends of the conductor will part sufficiently to extinguish the arc and to arrive at an open-circuit condition. This would then enable an open-circuit protection device to operate. If this protection was not present, one has then to assume that the two ends will somehow come together again for the arc to re-start. This extremely unlikely scenario is the only way that an open-circuit protection device will be of use in this situation.
2.3 Detection of Arcs Between Components
There remains the problem of dealing with an arc between, say, a cable and an electrode if the connection has parted but remains in close proximity. Such a fault is neither a current to earth nor is it an open circuit. To protect against such a fault requires a circuit which will detect the difference between the normal secondary current and one which includes an arc. A circuit to do this has been developed and its operation will be described in Section 13.
Earth-leakage protection applied to the high-voltage circuit between a transformer and a neon sign should not be confused with similar protection for the mains supply circuit, which is fitted in accordance with the requirements of the IEE Wiring Regulations. The latter protection is designed to switch off the mains supply to a circuit in the event of an earth fault in that supply.
A similar fault occurring in the high-voltage output circuit of a sign transformer or high-frequency ballast will result in only a small increase in the mains-supply current to that transformer or ballast, and this will not be registered by a protective device installed in the mains supply. For this reason, means of earth-leakage protection which sense earth fault currents in the high-voltage circuit are needed.
Short circuits to earth in the high-voltage circuit can result in electric arcs which may cause fires. Tests have shown that an arc to earth having a current of only 15 mA will start a fire if it impinges on the edge of an acrylic sheet for a few seconds.
If there is a connection between a high-voltage conductor and the surface of flammable material (for example an electrode assembly has broken away and fallen against that surface) the surface may be damp enough to allow conduction to earth (tracking). If the resulting current is large enough to raise the temperature of the surface above the point where it decomposes, one of the results may be a residue of carbon or other conducting material. This will allow the current to increase until a level is reached where the material ignites.
Earth-leakage protection for high voltage sign circuits is specified in limited circumstances in BS 559 but will be required for all circuits when the European Sign Specification (EN 50107) is published. A device which measures
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& GRAPHICS an earth fault current in the high-voltage output circuit and causes the mains supply to that current to be switched off will normally be used with standard sign transformers. Equivalent protection for sign circuits supplied from high-frequency ballasts is usually included within the ballasts operating circuit. Ways in which this is achieved are discussed below in Section 10.
Note: The characteristics for an earth-leakage protective device are specified in EN 50107, and the Standard requires that it should operate within 200 ms in the event of an earth fault current exceeding 25 mA. These characteristics are adequate to ensure operation before an electric arc can start a fire. They are, however, not intended to ensure protection of persons against the effects of electric shock.
There are many ways of providing earth-leakage protection for sign transformers but the basic principles are illustrated by the design shown in Figure 1. This is the type of device which is most frequently used in Britain.
The circuit shows a transformer “T1” supplying power to a typical load of four tubes. The secondary winding of the transformer includes a centre tap “C” which would normally be connected to earth. In this circuit it is connected to earth via the sensor terminals “3” and “4” of an earth-leakage protective device “Z”. Under normal operation there is very little impedance between these terminals, and point “C” can be considered at earth potential.
Unless there is a fault, the current “I1” which flows from high-voltage terminal “A” will be the same as the current “I2” which flows back into terminal “B”. There is no current “I3” flowing in the earth lead and the earth-leakage protection is inoperative.
If a fault occurs at point “D”, this will allow a current “I3” to flow to earth. The circuit for this fault current is completed by the return connection from earth to the centre tap of the secondary winding (point “C”) and this means that the same current “I3” will flow through the sensor circuit of the protective device. As long as this fault current exceeds the operating current of the device, it will operate the trip in a time of about 200 ms and open the contacts “X” and “Y”. These contacts switch off the mains supply to the transformer and remain open until reset.
Early forms of earth-leakage protection applied to the mains supply included sensors which were connected in the earth path, and across which a voltage was developed whenever there was a fault to earth in the mains supply. A voltage of about 40 V was required to operate the protective device and switch off the mains supply.
The presence of this voltage, even briefly, on the earth wiring of an installation was considered to be a hazard, and this led to the development of current-operated devices. Current transformers in the line and neutral can detect a small difference between the currents in those lines, and this difference may be assumed to be due to an earth fault.
Similar protection can be applied to high-voltage sign circuits by fitting current transformers in the output leads of the transformer. Of course, such transformers need to be insulated to the full no-load output voltage to earth, but this is fairly easily achieved by passing the high-voltage cables through the encapsulated transformers. Thus the high voltage cable acts as a single turn primary winding, linking with a completely isolated magnetic core and secondary winding.
As can be seen from the circuit in Figure 2, lamp currents “A” and “B” are normally the same so the outputs from current transformers “X” and “Y” are also the same and the operating circuit “Z” does not see a difference in the currents. However, if an earth fault “C” develops, this current will subtract from the current “B” and a difference will be measured by the operating circuit. By means of suitable amplification, this difference can be used to energise a switch “W”, disconnecting the mains supply.
This circuit is more complicated than the basic arrangement shown in Figure 1 and the extra complexity may affect the reliability of the arrangement. Also it is very difficult to use one device to protect more than one lamp circuit in the manner shown in Figure 5. On the plus side, there is no need to have a transformer with a detachable link between the centre tap of the secondary winding and the earth point, and the same basic arrangement may be used to detect open-circuit conditions and series arcs, both of which are discussed below in
Sections 12 and 13.
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8.1 Transformer Connections
A practical circuit consisting of a sign, transformer and earth-leakage protection is shown in Figure 3. Note that the earth-continuity wiring “W7”, “W8” and “W9” is connected to the earth terminal of the transformer in the normal way. The terminal connected to the centre tap of the transformer output winding “E” is normally connected to the earth terminal by means of a wire link. This link must be removed for correct operation of the earth-leakage protective device.
In some transformers, the sensor to operate the earth-leakage protective device may be fitted within the transformer and connected between the “E” and earth terminals. A typical circuit is shown in Figure 4. Such transformers, and their integral sensors, are designed to work with specified protective devices and they should not be used with a separate earth-leakage protective device unless the two are guaranteed to work together . If in doubt, the presence, or otherwise, of a sensor should be checked by measuring continuity between the “E” and earth terminals after the fixed link has been removed. Testing with the meter connected one way round and then with reversed polarity should indicate an open circuit in both tests. If the transformer includes a sensor
(usually a zener diode) then there should be a low resistance in one direction.
Note: The meter should have an internal battery delivering at least 10 V. If this voltage is less than the forward break-over voltage of a zener diode, a high forward resistance might still be measured.
8.2 Connections to the Protective Device
The centre tap of the transformer output winding “E” is connected to the sensor input terminal of the protective device (marked “RE”). Although the voltage on this cable will not exceed about 10 V, the cable insulation should be rated for 240 V to ensure good mechanical strength. A suitable single-core cable would have a 1.0 mm2 conductor and 240 V grade insulation, coloured black. Alternatively, a fourth core in a 4-way flexible or 4-way
M.I.C.C. cable would be suitable.
Note: The earth terminal of the transformer, which is connected to its case or frame, must still be connected to the earth terminal of the mains supply.
8.3 Siting of the Protective Device
The device may be installed in any suitable location. For outdoor applications, the protective device should be housed within a weatherproof enclosure with a degree of protection corresponding to IP 54 of BS EN 60529.
Since there is no restriction on the length of leads connecting the protective device to the transformer, a suitable place might be close to the sub-circuit fuses feeding the sign(s). This would allow one device to be connected into the feeds to a number of transformers (see section 9 below). It would also enable the device to be reset by local personnel in the event of nuisance tripping.
Provided the switching contacts have an adequate rating, a single earth-leakage protective device may be used with a number of transformers. A typical circuit is shown in Figure 5. In order to avoid nuisance tripping, caused by the capacitive current into metal sheathed cable, no more than three transformers should be connected to each protective device and the total length of such cable should not exceed 30 metres. If unscreened silicone rubber cables are used for the connections between the transformers and tubes, the number of transformers connected to each protective device may be increased to six. In this case, there is no restriction on the length of the high-voltage cable.
On each transformer the centre tap of the secondary winding should be disconnected from the earthed case and onnected together, via cable E, to the sensing circuit of the protective device. This connection should be made using 1.0 mm2 cable with 330 V PVC insulation, coloured black. The other side of the sensing circuit should be earthed.
All other parts of the circuit should be wired in accordance with BS 559 and the IEE wiring regulations.
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With a high-frequency ballast a circuit to switch off the output voltage may be incorporated within the drive circuit to the power transistors. Once the drive is switched off, the transistors will cease conduction and there will be no output from the transformer.
There are many ways in which this may be done, one of which is shown diagrammatically in Figure 6. A sensor circuit “P1” is shown connected in the earth lead to the centre point of the transformer secondary winding. If a short to earth should occur in either high-voltage lead “A” or “B”, then the fault current will be sensed by “P1” which can be arranged to provide a disable signal to turn off the drive circuits.
Apart from safety reasons, it is important for the reliability of high-frequency ballasts that they do not operate under continuous open-circuit conditions and some manufacturers include such protection circuits within their units. One interesting way of doing this, which also doubles as an earth-leakage detection device, is shown in
Figure 7.
In this circuit, a voltage from each high-voltage output terminal (“A” and “B”) is fed back, via capacitors C1 and
C2 to a bridge circuit controlling the drive oscillator. This arrangement can detect a prolonged open-circuit condition and, by virtue of the resulting voltage imbalance between the two halves, can also detect an earth leakage. It will be noted that capacitors C1 and C2 have to withstand the full no-load voltage of the output transformer and have a rather special construction.
Note: It should be noted that, for an imbalance to occur in the event of an earth fault, the two halves of the output transformer secondary winding need to be magnetically isolated.
11.1 Reasons for the Non-Operation
The specification for earth-leakage protective devices requires that they operate in the event of an earth fault occurring where the resulting fault current exceeds 25 mA. It is assumed that earth faults, which result in part of the transformer secondary windings being short-circuited, will cause a current in excess of this value to flow through the sensor. Special devices are required to protect transformers with a lower current rating than 25 mA.
However, this assumes that after the fault has occurred the tubes will extinguish. If the tube operating voltage is on the low side for the no-load output voltage of a particular transformer, then it is possible for those tubes to remain alight on half the original output voltage. A typical circuit with a current measuring device “Z” connected in the earth lead is shown in Figure 8.
The circuit assumes that, in event of a short circuit to earth from either high-voltage lead, the tubes will extinguish and the sensor will measure the current to earth. If the tubes should stay alight, the residual current
“B” through those tubes will subtract from the current “A” flowing to earth through the short circuit, and the resulting current “C” may be insufficient to operate the earth-leakage protection system.
11.2 Practical Examples of Non-Operation
Consider, for example, a typical 10 kV, 25 mA transformer which has a short-circuit current to earth of approximately 28 mA. This should be sufficient to operate a standard earth-leakage protection device. With a variable load consisting entirely of neon tubes, having tube operating voltages from 7 kV down to about 4.3 kV, the tubes extinguish when an earth fault is applied and the expected short-circuit current is obtained through the sensor. Below 4 kV the tubes remain alight and the tube current then subtracts from the fault current. The current through the earth-leakage sensor is reduced below its operating level and the device will not work. At a tube voltage of 3.5 kV, the sensor current is reduced to 14 mA, and at 1.7 kV it is reduced right down to 7 mA.
With mercury tubes, it is possible to arrange for a tube load of 4.9 kV to remain alight, although at a very low current. Even so, the sensor current is reduced to 21 mA and it is doubtful whether standard units would operate. At a voltage of 3.7 kV, the sensor current is down to 15 mA and no 25 mA device would work.
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11.3 Correct Loading of Transformers
It might be argued that the sign maker should ensure that the transformers are always loaded close to their maximum voltage. However, signs are rarely that convenient, and a spread of tube loads must be allowed for. It is suggested that the tube operating voltage is not less than 50 % of the transformer no-load output voltage for mercury tubes and not less than 40 % for neon tubes. Alternatively, the circuit modification described in Section
11.4 should be considered.
11.4 Splitting the Output Circuit
Caution: The following suggested circuit modification may be applied to all Euro-pattern transformers manufactured by Tunewell Transformers Ltd. and to recent F.A.R.T. transformers supplied by Amari Plastics Ltd.
It must not be applied to any transformer where the two halves of the secondary winding are not magnetically separated.
Since the secondary windings of a typical neon transformer are magnetically separated, it is possible for each half secondary to operate half the tube load independently of the other half. One way of doing this is shown in Figure 9. In this diagram, a transformer with secondary windings “X” and “Y” is shown operating four tubes.
The sensor of an earth-leakage protective device is shown connected between the centre tap of the winding “E” and earth.
The modification consists of connecting a cable “W” between the series connection half way along the line of tubes (in this case between tubes “B” and “C”) and the “E” terminal of the transformer. If an earth fault occurs
(shown here as the connection “Z”), then tubes “C” and “D” must extinguish and the whole short-circuit current of half winding “Y” will flow through the sensor.
Meanwhile, the half secondary winding “X” will continue to supply the tubes “A” and “B” with the normal tube current which will flow along wire “W” bypassing the sensor circuit. In this manner, the problems of nonoperation of earth-leakage protective devices is avoided.
Although this arrangement is best suited to circuits having an even number of equal length tubes, it may be applied to other circuits provided that the point chosen for the connection is not normally operating at a voltage far removed from earth. In, say, a circuit having five tubes, if the connection is made between tubes 3 and 4, then the three-tube circuit will be operating at a lower current than the two-tube circuit. Since modern transformers provide a reasonably constant current output, the difference in brightness will probably not be noticed. However, care should be taken to ensure that the voltage on each half secondary is still sufficient to operate its particular tube load and that the tube operating voltage still lies within the tolerances specified by the transformer supplier.
The cable “W” should have normal 250 V grade insulation, coloured black. For mechanical strength (not currentcarrying capacity) the conductor size should be at lease 1.0 mm2.
The use of a circuit to switch off the mains supply to a sign, or otherwise disable the output, in the event of an open circuit will provide additional safety, particularly for signs which are installed within arm’s reach. If an open circuit occurs during the time the circuit is operating, e.g. if a tube is broken, then it is essential that the circuit is switched off very quickly. Devices will be required to operate within 22 ms.
However, the operation of the device needs to be delayed when a lamp circuit is switched on, other wise it will switch off the circuit if the lamps are slow to start. On cold mornings some lamps may take one or two seconds to start. To cater for this situation, the European Standard will require a delay of not less than 3 seconds and not more than 5 seconds before the protective circuit is operational.
The basic circuit is very similar to that shown in Figure 2. Instead of detecting a difference between the lamp currents “A” and “B”, the operating circuit is set up to react when neither currents “A” nor “B” are present. As will be obvious, it is a matter of straightforward design to combine the detection of both open-circuit and earth-fault conditions in an arrangement similar to that shown in Figure 2. For high-frequency ballasts, a bridge circuit similar to that shown in Figure 7 may be used to provide an indication that the full no-load output voltage is still present.
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There remains a problem of identifying an arc which forms when a gap is introduced in a high-voltage circuit. If, for example, a wire is detached from an electrode but the end still remains close to that electrode, then an arc may take place between that wire and the electrode terminal and this might be the cause of a fire. Since there may be no current flowing to earth and since the tubes will still be alight, this fault cannot be detected by either earth-leakage or open-circuit protective devices.
Of course, with an arc in the circuit the current is quite different from that normally flowing through the lamps.
As might be expected, the normally smooth near sinusoidal, lamp current is changed to a waveform having a multitude of spikes. owever, it turns out to be quite difficult to detect the difference in mean levels between this and the normal waveform by electronic means. The problem is solved if the two waveforms are differentiated, the normal sinusoidal waveform remains sinusoidal (just with its phase shifted by 900) but the sharp edges of the waveform with the arc produce a series of high peak values, equivalent to the slope of the original waveform.
The difference in mean value between the two differentiated waveforms is thus quite large and easily detectable.
The actual circuit to do this uses current transformers similar to those shown in Figure 2. The operating circuit differentiates the lamp current waveform and then measures the mean value of this differentiated waveform at
150 ms intervals. By constantly comparing the mean value with the one that occurred 150 ms before, the change which takes place when an arc occurs in the circuit is quickly detected. Thus switch-off can take place in about
150 ms after the arc has started - much less time than it takes for that arc to start a fire.
Devices to do this exist in the prototype stage and are likely to be available within the near future. Members wishing to use this form of active protection should contact the British Sign & Graphics Association who will provide the name of the company carrying out this development.
If insulation or clearances between high-voltage connections and earth are inadequate it is still possible for a circuit to operate normally when it is dry. However, when those connections become wet, the moisture may bridge the gap and allow an earth fault current to flow, and this may cause an earth-leakage protective device to trip. If spurious tripping is to be avoided, all high-voltage connections should be checked and inadequate insulation or clearances rectified. The following points should be considered: a) Every high-voltage connection should be inspected to see whether: i) It is protected by insulation sleeves as specified in BS 559 in situations where its use is required by that Standard.
Note: This means the use of silicone rubber sleeves in installations where both the faces and the sides of a sign are constructed from flammable material.
ii) The clearances and creepage distances to earthed metal or surfaces likely to become conducting when wet are greater than the minimum specified in BS559.
Note: In a circuit supplied by a 10 kV transformer (5 kV to earth) the minimum clearance should be 20 mm and the minimum creepage distance should be 50 mm.
iii) The minimum creepage distance and clearance between the connection and flammable materials is 50mm.
b) The position of tubes with respect to flammable materials should be inspected to see whether: i) The distance between a point on the tube close to an electrode and the flammable material is not less than 15mm.
ii) The distance between any other point on the tube and flammable materials is not less than 10mm.
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To ensure that open-circuit or earth-leakage protective devices have been installed correctly and are operating satisfactorily, it is essential that the circuits are tested after installation. It is also important that the tests are repeated during subsequent maintenance visits. The purpose of the tests is not to check the performance characteristics of the devices (e.g. 25 mA in 200 ms). Those parameters should be certified by the supplier of the device. The sign installer should ensure that suitable tests, which may be carried out relatively simply on site, have been specified by the supplier of the earth-leakage or open-circuit protective device.
The British Sign & Graphics Association has used its best endeavours to ensure that the information given in these Technical Guidelines is accurate but it cannot accept responsibility for loss occasioned to any person acting or refraining from action as a result of the material in these guidelines. If in any doubt specialist advice should be obtained.
The content of this guideline is copyright to the British Sign & Graphics Association and may not be re-produced without the consent of the Association’s Council.
Figure 1: Basic circuit for earth-leakage protection
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Figure 2: Protective circuit using current transformers
Figure 3: Practical circuit for earth-leakage protection
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Figure 4: Example of a sensor connected within a transformer
Figure 5: Protection of multiple transformers
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Figure 6: Typical earth-leakage protective circuit for a high-frequency ballast
Figure 7: Fault detection using a bridge circuit
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Figure 8: Problems with earth-leakage detection
Figure 9: Additional connection to ensure operation of earth-leakage protection
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