Western Telematic Surge Protector TAN 1005 User Manual

991.115  
03/99  
Application Note  
TAN 1005  
Surge Suppression  
for Zone 0 Locations  
Synopsis  
This note discusses the surge  
protection requirements of  
intrinsically safe circuits entering a  
Zone 0 hazardous area. It  
analyses the potential gradients  
generated by lightning strikes and  
their possible routes of invasion.  
The alleviation of the problem at  
the zone 0 interface transfers the  
problem elsewhere and an  
adequately safe pragmatic  
solution is proposed.  
A member of The MTL  
Instruments Group plc  
Telematic  
 
tal requirement of all the methods of protection used for power equip-  
ment. It is not usual for lightning induced current to directly cause enough  
heating to create a hazard by temperature ignition, since the current  
pulses are too short to create a sustained high temperature. However,  
vapourisation of flimsy conductors such as printed circuit tracks is not  
unusual. Overheating may occur if there is a power follow through of  
a fault initiated by the lightning induced voltage. It can be argued that  
if intrinsically safe apparatus is likely to be subjected to lightning dam-  
age then it is necessary to protect it since, following the lightning dam-  
age, its intrinsic safety may be impaired. There is no requirement in the  
apparatus standard to consider the effect of excessive surges, which  
are difficult to predict and could lead to damage. The problem should  
not be exaggerated, since lightning damage usually results in failure  
to a safe condition and also to operational failure and hence should  
be noticed and corrected. Possibly the need to repair or remove non-  
functional electrical equipment needs to be given further emphasis in  
the code of practice.  
SURGE SUPPRESSION FOR  
ZONE 0 LOCATIONS  
1
INTRODUCTION  
For many years there has been general recognition that there is a  
significant problem from lightning strikes on installations such as stor-  
age tanks. The codes of practice for instrumentation in hazardous ar-  
eas for Germany and Holland both contain recommendations for spe-  
cific installation practice. In the United Kingdom the code of practice  
contains no detailed requirements and the problem has always been  
approached on an individual installation basis. Perhaps the clearest  
references are in the draft revision of the IEC code which contains two  
specific references to lightning problems. These, together with the rel-  
evant clause on potential equalisation, are quoted in full as an appen-  
dix (clauses 6.3, 6.5 and 12.3).  
It is accepted that transient hazards during infrequent electrical faults  
can occur in Zones 1 and 2 providing that they are removed as quickly  
as is practical. The argument being that the coincidence of the poten-  
tially hazardous electrical fault and a flammable mixture of gas is suf-  
ficiently improbable to be acceptable. In the particular case of light-  
ning a similar analysis suggests that transient hazards caused by points  
of lightning impact and the occasional failure to bond adequately are  
possibly acceptable in Zone 1 and 2 but not acceptable in Zone 0.  
Fortunately the majority of Zone 0 locations are contained within proc-  
ess vessels which form an adequate Faraday cage which effectively  
prevents significant potential differences within the Zone 0 and hence  
the problem is generally controllable. Where problems are known to  
exist then special precautions are taken, for example the bond be-  
tween the floating roof of a storage tank and the tank itself is designed  
with considerable care, and subjected to frequent inspections. A prob-  
lem is introduced when the Faraday cage of the Zone 0 is broken by  
the introduction of equipment for measurement purposes.  
Although this code of practice has not yet been finally voted on and  
published it is likely to form the basis of accepted practice in signifi-  
cant parts of the world and forms a convenient reference document.  
When a plant is struck by lightning then the point of impact would  
inevitably ignite a gas and air mixture that was present. Ignition at  
points other than the point of impact are dependent on the efficiency of  
bonding which must be adequate to prevent side flashes and hence  
bonding should have a low impedance as well as a low resistance.  
The majority of petrochemical installations are adequately bonded and  
sufficiently robust to prevent excessive lightning damage although some  
side flashes usually occur following a significant adjacent strike. Co-  
rona discharge from structures does occur in some atmospheric condi-  
tions and multiple streamers rising from structures to meet the usual  
lightning downward leader (which selects one of them) are a well es-  
tablished phenomenon. It is possible that if either a lightning flash, an  
upward corona streamer, or a side flash pass through a flammable  
mixture of gas then ignition will occur. In general, conventional bond-  
ing of a plant is considered adequate and the implications of possible  
lightning impact points are not considered a significant problem ex-  
cept in the case of vents which frequently discharge. Where lightning  
can damage the electrical insulation of power circuits there is a tran-  
sient potential hazard caused by the follow through of the power cir-  
cuit. This should however be rapidly removed by the electrical protec-  
tion ie. fuses, out of balance circuit breakers etc. which is a fundamen-  
Figure 1 shows an average contents temperature gauge being used in  
a storage tank and this illustrates the problem. The potential equalising  
network is shown diagramatically as a substantial structure intercon-  
nected electrically, in practice it is the plant structure bonded together.  
The transmitter protruding from the tank top is intended to illustrate the  
concept. In practice in a high lightning activity area it would be unwise  
to have the equipment protruding from the tank in this way since it  
would possibly invite a direct strike and could be the natural source of  
corona discharge. It should be provided with some mechanical protec-  
L1  
100kA  
10µS  
L2  
30KV  
30kV  
Computer 0V  
Power 0V  
Potential equalising  
network  
10m  
0.1µH/m  
(10kV)  
0.1µH/m  
10kA  
500m  
(50kV)  
Figure 1 Installation without surge protection  
1
 
tion from this possibility or sited with care in the shelter of some other  
protrusion. The diagram shows a two wire 4 to 20 milliamp transmitter  
with internal isolation fed from a galvanic isolator. To establish the  
order of the problem some assumptions are made which cannot be  
fully justified but are believed to be reasonable. These are:  
When the apparatus standard was being written the question of the  
quality of the insulation of the circuit from earth was discussed. It was  
decided that except where the intrinsic safety was critically dependent  
e.g. where a current limiting resistor could be short circuited, then the  
creepage and clearance requirements should be waived but that the  
measure of insulation adequacy was a 500 volt insulation test. This  
has led to occasional problems e.g. strain gauges, but in general has  
not caused problems. It was not thought that circuits would be sub-  
jected to 500 volts in the hazardous areas, if they are, then they are no  
longer intrinsically safe. [Note - Using 20 microjoules as the ignition  
energy of hydrogen, the permissible capacitance associated with 707  
volts is 80 picofarads and the safe voltage corresponding to the per-  
mitted 10 nanofarads is 63 volts]. The subsequent analysis therefore  
ignores the 500 volt insulation test requirement and concentrates on  
producing a solution which reduces the voltages applied to the Zone 0  
in transient conditions to an acceptably safe level.  
a) The tank has an inductance of 0.1µH/metre and is 10 metres high  
before reaching the equipotential plane of the plant.  
b) The potential equalisation system has a similar low inductance of  
0.1µH/metre and the tank is situated some 500 metres from the  
control room.  
c) Lightning strikes the tank, and the strike is 100kA rising linearly in  
10 microseconds. Some 10kA is assumed to flow through the po-  
tential equalising bond to the control room distribution centre trans-  
former.  
With these assumptions the transient peak volts across the tank is 10kV  
and the voltage across the potential equalising network is 50kV. The  
resultant 60kV potential difference would be divided across the isola-  
tion within the interface and the isolation within the transmitter with a  
high probability that both would break down.  
3
CERTIFICATION OF SURGE SUPPRESSORS  
Usually, surge suppressor circuits can be classified as “simple appara-  
tus” using any of the available definitions. Fortunately the requirements  
of simple apparatus have been more clearly defined in the second  
edition of EN50020 (reproduced in Appendix 2) and hence due al-  
lowance for the small inductors sometimes used can now be made.  
This example is used to illustrate the remainder of this document. In  
practice all specific installations will differ in detail from this example  
but the general principles are illustrated by this analysis. Usually a  
document of conformity for the intrinsically safe system in accordance  
with EN50039 should be generated for the specific system. The com-  
bination of MTL Instruments Ltd and Telematic Ltd is particularly suited  
to giving assistance in creating such documentation, should help be  
required.  
It is normal practice to have “simple apparatus” certified by an appro-  
priate body such as BASEEFA if they are frequently used in intrinsically  
safe circuits. Although not strictly essential such third party certification  
gives additional comfort to the end user and makes the marketing of  
the product easier. It is important however to recognise that the certifi-  
cation relates only to the effect the surge suppression device has on the  
intrinsic safety of the circuit when the circuit is not affected by lightning  
transients. There are no requirements in the apparatus standards relat-  
ing to the performance of surge suppressors. Although BASEEFA do  
satisfy themselves that the product they are certifying is not useless they  
are not responsible for its performance during a transient surge, nor is  
anyone able to claim that the circuit is intrinsically safe during the brief  
time it is affected by the lightning surge. The full implications of the  
recent “ATEX” directive with respect to surge suppressors has not yet  
been pursued, but may lead to some additional testing requirements.  
2
INTRINSIC SAFETY REQUIREMENTS  
FOR EARTHING AND BONDING  
Usually instrumentation introduced into a Zone 0 is intrinsically-safe to  
the ia requirements and is nearly always ia IIC T4 certified by some  
appropriate organisation. If this simplifying assumption is made then  
certain aspects of intrinsic safety practice need to be examined with  
this application in mind.  
In the IEC draft code of practice a strong preference for using galvani-  
cally isolated interfaces for Zone 0 is expressed. The arguments for  
galvanic isolation have always been strongly advocated within Ger-  
many and France and are based on the assumption that galvanically  
isolated circuits are less susceptible to earth faults and potential differ-  
ences between earths than shunt-diode safety barriers. There are liter-  
ally millions of circuits using shunt-diode safety barriers and although  
there have been a number of operational problems, there is no indica-  
tion that any safety problem has arisen from their use and hence prob-  
ably the arguments are theoretically correct but may not be practically  
significant. However the economic difference between shunt-diode safety  
barriers and isolators is not significant in this type of installation and if  
necessary high accuracy transfer can usually be achieved using dig-  
ital signals. Although an acceptable solution using shunt-diode safety  
barriers can be achieved, this analysis proceeds on the assumption  
that isolated interfaces will be used if only to avoid the distraction of  
any argument resulting from the use of shunt-diode safety barriers.  
TP48  
+
300V  
2µS  
It is usual to require that intrinsically safe circuits are fully floating or  
earthed at one point only. The reason for this requirement is to prevent  
significant circulating currents flowing within the circuit due to poten-  
tial differences within the plant. The problem is not so much that there  
is a significant safety risk but that it is difficult to certify a system with  
unspecified currents. In practice the safety analysis carried out with  
multiple earth faults is based on the assumption that all earths are at  
the same potential and interconnected by zero impedance. Since the  
single earth philosophy is largely compatible with the low frequency  
interference avoidance practices in instrumentation this has not been  
challenged until recently. The increased awareness arising from the  
EMC directive of the effects of high frequency interference has led to  
the greater use of decoupling capacitors on input circuits which are a  
form of multiple earthing. This is recognised in both the apparatus  
standard and the code of practice, the latter permitting a total capaci-  
tance of 10nF in any one circuit.  
60V  
L1  
L2  
Instrument  
housing  
60V  
60V  
60V  
Tank shell  
Figure 2 Surge suppresion of the transmitter  
2
 
60V  
60kV  
TP48  
Bonding strap  
10kV  
50kV  
Figure 3 System with transmitter only protected  
at 1.5kV rms) but damage would be expected at 60kV. The usual result  
of this failure would be damage to the computer interface which would  
have both cost and operational safety implications. In non hazardous  
locations it is not unusual for the loss of individual transmitters to be  
accepted as sacrificial but to protect the computer interface so that the  
possibility of more complex interacting faults is reduced, and the pos-  
sibility of the total system being shut down is removed.  
4
PROTECTION OF THE SENSOR AND  
TRANSMITTER  
The problem of surge protection of the transmitter and sensor is rela-  
tively easy to solve since it is only necessary to prevent significant  
voltage differences so as to avoid ignition capable sparks. This can be  
achieved by using a combination of surge limiting devices, which ef-  
fectively control the voltage between the signal wires and with respect  
to the adjacent structure.  
The suppressor discussed has a BASEEFA certificate which permits its  
use in conventional intrinsically safe circuits [it is also Ex d certified].  
The level of protection offered has been carefully chosen so that all  
known two wire transmitters can be adequately protected. The leak-  
age currents associated with shunt protection devices are controlled so  
that they do not significantly affect the operational accuracy of the loop.  
A practical solution to this problem is to use a Telematic TP48 (see  
figure 2) which contains the necessary parallel surge components in  
an encapsulated block within a stainless steel hexagon bar which can  
be screwed into the unused cable entry of the transmitter. To achieve  
suppression against the expected transients it is necessary to use a  
combination of gas discharge tubes and solid state devices. With the  
usual test waveform this combination restricts the transient voltage be-  
tween the circuit and structure to 300 volts which then falls to 60V after  
two microseconds and the voltage between the signal lines to 60V. It is  
a matter of some debate as to what transient voltages would be antici-  
pated on a practical installation with protection but they would not  
exceed 150V and almost certainly would be considerably less.  
5
PROTECTION OF THE GALVANIC  
ISOLATOR AND SAFE-AREA EQUIPMENT  
The use of surge suppression between the isolator and the computer  
input interface protects the computer interface and the isolators are  
then sacrificial. The unspecified damage to the isolators is not however  
desirable and the better installation is to protect the isolators on the  
hazardous area side as indicated in figure 4.  
To be effective the surge suppressor must be adequately bonded to the  
structure. Almost all transmitters contained within metallic enclosures  
have both internal and external bonding connections which can be  
utilised to ensure adequate bonding. The need for the external bond is  
reduced if the mounting of the transmitters ensures an effective bond.  
but if there is any doubt a substantial bond should be used. The size of  
the bond is largely determined by the need to be mechanically robust.  
A flat short braid with suitable tags has much to commend it.  
The standard solution to this problem is to use the SD32X which would  
reduce the voltages applied to the isolator to the acceptable levels as  
indicated and would not significantly affect the operation of the circuit.  
[Note. There is a version of the suppressor which has a replaceable  
fuse and isolation link. In this application the fuse it not likely to be  
blown hence this alternative should only be used if the isolation link is  
thought to be useful].  
This suppression circuit produces in the worst case condition a short  
150V pulse across the transmitter isolation and a longer 60V pulse,  
both of which the isolation will normally reject. Any small transient  
which is fed by the transformer capacitance to the sensor circuit would  
be absorbed by the high frequency input filter capacitors of the sensor  
input circuit.  
The SD series has not yet been certified by BASEEFA as being suitable  
for connection into intrinsically safe circuits although an application  
has been made and hence its acceptability is based on it being simple  
apparatus as defined in the second edition of EN50020 [see Appen-  
dix B]. It does contain two small inductors which have a combined  
inductance of 200 microhenries. However the conventional transmitter  
circuit is powered from a 28 volt 300 ohm source which has permitted  
cable parameters of 0.13 microfarads and 4.2 millihenries. The per-  
mitted length of cable is usually restricted to approximately 600 metres  
by the capacitance requirement and hence a marginal reduction of the  
permitted inductance to 4 millihenries (equivalent to 4Km) has no ef-  
fect.  
The results of fitting surge suppression on the transmitter therefore en-  
sures that there is an adequate level of protection for the sensor and  
transmitter. However removing the potential difference from the trans-  
mitter transfers the whole of the potential difference to the isolator as  
illustrated in Figure 3. Typically an intrinsically safe isolator will with-  
stand an occasional 5kV transient (the components are routinely tested  
3
 
1kA (10µS)  
15m  
15µH  
1.5kV  
Figure 4 Intrinsically safe circuit fully protected  
The system should be designed so that when the surge current is di-  
verted the voltage drop across the bonding conductor does not create  
a large voltage across the isolator. Figure 4 gives an illustration of a  
satisfactory system. With the currents and distances indicated the iso-  
lator is still subjected to a 1.5kV pulse and hence the importance of  
keeping the interconnection as short as possible cannot be over em-  
phasised.  
within the intrinsically safe circuit. During this short time the circuit is  
not intrinsically safe but the equipment at either end of the line is oper-  
ating within its rating. Any hazard which does exist is in the cable and  
is in the Zone 1, or Zone 2 location. It is a smaller hazard than that  
which would exist without the protection and hence is a desirable ac-  
ceptable solution.  
The use of a second suppressor on the circuit means that the intrinsi-  
cally safe system is now indirectly bonded at two points. The sequence  
in which the suppressors begin to conduct is quite complex since it  
does depend on how the potential difference between the two earths  
develops. The sustained situation which is the least desirable is that the  
transmitter protector requires 60 volts to conduct and the computer  
protector 30 volts to conduct. Hence there would need to be at least  
90 volts between the two earths before a significant current could flow  
6
PROTECTION OF SUPPLIES AND  
SIGNALS FROM EXTERNAL SOURCES  
If the mains supply to the system is subject to lightning surges then the  
operational integrity and safety of the system can be adversely af-  
fected. An obvious invasion route for the intrinsically safe system is via  
the isolator supply which is derived either directly or indirectly from the  
supply. The intrinsic safety certification process assumes that the power  
Signal Suppressor  
Data link  
Mains supply  
Mains Filter  
suppressor  
Figure 5 Adequately protected system  
4
 
supply will contain a significant amount of surges but if for any reason  
the supply is particularly exposed to invasion from lightning induced  
surges then some consideration to suppressing the main supply should  
be given.  
In installations with intrinsically-safe circuits for Zone 0 the intrinsically-  
safe apparatus and the associated apparatus shall comply with IEC  
79-11 category ‘ia’. Associated apparatus with galvanic isolation  
between the intrinsically-safe and non-intrinsically-safe circuits is pre-  
ferred. Associated apparatus without galvanic isolation may be used  
provided the earthing arrangements are in accordance with item 2) of  
12.2.4 and any mains powered apparatus connected to the safe area  
terminals are isolated from the mains by a double wound transformer,  
the primary winding of which is protected by an appropriately rated  
fuse of adequate breaking capacity. The circuit (including all simple  
components, simple electrical apparatus, intrinsically-safe apparatus,  
associated apparatus and the maximum allowable electrical param-  
eters of interconnecting cables) shall be of category ‘ia’.  
A practical economic solution is to protect the supply input to the com-  
puter system as indicated in figure 5.  
A similar argument can be made if a data link is made to any remote  
location. This is less likely to directly affect the intrinsically safe circuit  
but can be very damaging to the computer.  
7
CONCLUSION  
Simple apparatus installed outside the Zone 0 shall be referred to in  
the system documentation and shall comply with the requirements on  
IEC 79-11, category ‘ia’.  
The solution shown in figure 5 is therefore the best practical solution to  
achieve safety for circuits entering Zone 0 where there is a significant  
probability of the circuit being influenced by adjacent lightning strikes.  
If earthing of the circuit is required for functional reasons the earth  
connection shall be made outside the Zone 0 but as close as is reason-  
ably practicable to the Zone 0 apparatus.  
It is probable that this solution is not directly applicable to all installa-  
tions but a solution based on a similar analysis is usually achievable.  
MTL and Telematic are in an almost unique position to give advice on  
this problem and consider that they have the competence to assist in  
preparing the relevant documentation.  
If part of an intrinsically-safe circuit is installed in Zone 0 such that  
apparatus and the associated equipment are at risk of developing  
hazardous potential differences within the Zone 0, for example through  
the presence of atmospheric electricity, a surge protection device shall  
be installed between each non-earth bonded core of the cable and the  
local structure as near as is reasonably practicable, preferably within  
1 m, to the entrance to the Zone 0. Examples of such locations are  
flammable liquid storage tanks, effluent treatment plant and distillation  
columns in petrochemical works. A high risk of potential difference  
generation is generally associated with a distributed plant and/or ex-  
posed apparatus location, and the risk is not alleviated simply by us-  
ing underground cables or tank installation.  
APPENDIX A  
This appendix is comprised of extracts from the draft IEC 79-14 code  
of practice of electrical installations in hazardous areas dated October  
1994. It may still be modified in detail but it is not probable that the  
principles will change.  
6.3  
Potential equalisation  
Potential equalisation is required for installations in hazardous areas.  
For TN, TT and IT systems all exposed and extraneous conductive parts  
shall be connected to the equipotential bonding system. The bonding  
system may include protective conductors, metal conduits, metal cable  
sheaths, steel wire armouring and metallic parts of structures, but shall  
not include neutral conductors. Connections shall be secure against  
self-loosening.  
The surge protection device shall be capable of diverting a minimum  
peak discharge current of 10 kA (8/20 µs impulse to IEC 60-1, 10  
operations). The connection between the protection device and the  
local structure shall have a minimum cross-sectional area equivalent to  
4 mm2 copper.  
The spark-over voltage of the surge protection device shall be deter-  
mined by the user and an expert for the specific installation.  
Exposed conductive parts need not be separately connected to the  
equipotential bonding system if they are firmly secured to and are in  
metallic contact with structural parts or piping which are connected to  
the equipotential bonding system. Extraneous conductive parts, which  
are not part of the structure or of the electrical installation, need not be  
connected to the equipotential bonding system, if there is no danger of  
voltage displacement, for example frames of doors or windows.  
NOTE - The use of a surge protection device with a spark-over voltage  
below 500 V a.c. 50 Hz may require the intrinsically-safe circuit to be  
regarded as being earthed.  
The cable between the intrinsically-safe apparatus in Zone 0 and the  
surge protection device shall be installed such that it is protected from  
lightning.  
For additional information see clause 413 of IEC 364-4-41.  
Metallic enclosures of intrinsically-safe apparatus need not be connected  
to the equipotential bonding system, unless required by the apparatus  
documentation. Installations with cathodic protection shall not be con-  
nected to the equipotential bonding system unless the system is specifi-  
cally designed for this purpose.  
APPENDIX B  
Requirements of simple apparatus extracted from EN50020:1994.  
5.4  
Simple apparatus  
The following apparatus shall be considered to be simple apparatus:  
NOTE - Potential equalisation between vehicles and fixed installations  
may require special arrangements, for example, where insulated flanges  
are used to connect pipelines.  
a) passive components, e.g. switches, junction boxes,  
potentiometer and simple semiconductor devices.  
6.5  
Lightning protection  
b) source of stored energy with well defined parameters, e.g.  
capacitors or inductors, whose values shall be considered when  
determining the overall safety of the system.  
In the design of electrical installations, steps shall be taken to reduce  
the effects of lightning.  
NOTE - In the absence of IEC standards on protection against light-  
ning, national or other standards should be followed.  
c) sources of generated energy, e.g. thermocouples and photo-  
cells, which do not generate more than 1,5 V, 100 mA and  
25 mW. Any inductance or capacitance present in these sources  
of energy shall be considered as in b).  
Subclause 12.3 gives details of lightning protection requirements for  
Ex ‘ia’ apparatus installed in Zone 0.  
Simple apparatus shall conform to all relevant requirements of this stand-  
ard but need not be certified and need not comply with clause 12. In  
particular the following aspects shall always be considered.  
12.3  
Installations for Zone 0  
Intrinsically-safe circuits shall be installed in accordance with 12.2 except  
where modified by the following special requirements.  
5
 
1) Simple apparatus shall not achieve safety by the inclusion of  
voltage and/or current limiting and/or suppression devices.  
2) Simple apparatus shall not contain any means of increasing  
the available voltage or current, e.g. circuits for the genera-  
tion of ancillary power supplies.  
3) Where it is necessary that the simple apparatus maintains the  
integrity of the isolation from ‘earth’ of the intrinsically-safe  
circuit, it shall be capable of withstanding the test voltage to  
earth in accordance with 6.4.12. Its terminals shall conform  
to 6.3.1.  
4) Non-metallic enclosures and enclosures containing light met-  
als when located in the hazardous area shall conform to 7.3  
and 8.1 of EN50014.  
5) When simple apparatus is located in the hazardous area it  
shall be temperature classified. When used in an intrinsically  
safe circuit within their normal rating switches, plugs and sock-  
ets and terminals are allocated a T6 temperature classification  
for Group II applications and considered as having a maxi-  
mum surface temperature of 85°C for Group I applications.  
Other types of simple apparatus shall be temperature classi-  
fied in accordance with clause 4 and 6 of this standard.  
Where simple apparatus forms part of an apparatus containing other  
electrical circuits the whole shall be certified.  
The principal author of this application note is L C Towle. BSc CEng. Chairman of Telematic Ltd. All Telematic Application Notes  
have significant input from the staff at Telematic and its customers. If you have any comments (preferably constructive) on this document,  
please make them to the author so that the document can be amended and made even more useful.  
Telematic Limited  
Alban Park, Hatfield Road, St Albans, Herts AL4 0XY  
Telephone +44 (0)1727 833147 Fax +44 (0)1727 850687  
Telematic  
A member of The MTL Instruments Group plc  
 

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