REPORT INTO THE INCIDENT AT MUTTRAH OLD
STORES PRIMARY SUBSTATION
28 MARCH 2006
OCTOBER 2006
TABLE OF CONTENTS
GLOSSARY OF TERMS .................................................................................................. 3
1
INTRODUCTION AND SUMMARY OF FINDINGS .................................................. 5
1.1
Summary of Findings ..................................................................................................... 5
1.2
Recommendations ......................................................................................................... 6
2
SCOPE OF INVESTIGATION AND METHODOLOGY ............................................ 7
2.1
Summary of the Incident ................................................................................................ 7
2.2
Scope of investigation .................................................................................................... 7
2.3
Methodology................................................................................................................... 9
3
BACKGROUND – RETROFITTING OF SWITCHGEAR........................................ 10
4
RETROFITTING AT MUTTRAH OLD STORES PRIMARY SUBSTATION........... 12
4.1
Substation Design and Layout ..................................................................................... 12
4.2
The Contract for Retrofitting at Muttrah ....................................................................... 14
5
SEQUENCE OF EVENTS....................................................................................... 17
6
TECHNICAL INVESTIGATION............................................................................... 20
6.1
The KEMA Report ........................................................................................................ 20
6.2
The ABB Italy Report ................................................................................................... 25
6.3
Cause of the Incident ................................................................................................... 27
6.4
Technical Conclusions ................................................................................................. 28
7
THE CONTRACT .................................................................................................... 30
7.1
Placing of the Contract................................................................................................. 30
7.2
The Contract – Conclusions......................................................................................... 31
8
SITE SAFETY ......................................................................................................... 32
8.1
Use of Limitation of Access.......................................................................................... 32
8.2
Conclusions – Site Safety ............................................................................................ 32
9
FIRST AID............................................................................................................... 34
10
RECOMMENDATIONS....................................................................................... 35
10.1
Retrofitting .................................................................................................................... 35
10.2
Contracts ...................................................................................................................... 35
10.3
Site Safety .................................................................................................................... 35
2
GLOSSARY OF TERMS
The terms defined in this glossary refer specifically to the apparatus/equipment involved
in the incident.
Busbar
An electrical conductor that makes a common connection between several Feeders
or Circuits
Bushing
The insulating material surrounding the fixed Busbar and Feeder Contacts in the
Cubicle.
Circuit Breaker
A device designed to open a Circuit either by manual action for the purpose of
isolation or by automatic action when current exceeds a value larger than permitted.
Contacts
Junction of electric conductors. The female Contacts (known also as ‘tulips’) on the
Truck engage the male Contacts in the Cubicle when the Circuit Breaker is
Racked-In to the Service Position
Cubicle or Panel
The fixed portion of a Switchboard. Contains the fixed Busbar and Feeder
Contacts together with the protection and metering devices.
Feeder or Circuit
The cable feeding a particular substation or location from a Switchboard
Flashover
An electrical discharge between one phase and another or between one phase and
ground, sometimes referred to as a short circuit.
Insert
The process of pushing the Truck horizontally into the Cubicle
Oil Circuit
Breaker
A Circuit Breaker using oil as the insulating medium
Rack-In
The process of raising the Circuit Breaker to the Service Position with the Feeder
and Busbar Contacts engaged.
Service Position
A Circuit Breaker Truck is in the Service Position when the moveable Contacts
on the Truck are engaged into the fixed Contacts in the Cubicle
3
SF6 Circuit
Breaker
A Circuit Breaker using Sulphur Hexaflouride as the insulating medium
Spouts
Name commonly used to describe the combination of Bushing and Contacts in the
Cubicle.
Switchboard
A Switchboard is formed by a collection of Cubicles and their associated Trucks.
Truck or Carriage The moveable portion of a switchboard. Contains the Circuit Breaker and the
Busbar and Feeder moving Contacts.
Vacuum Circuit
Breaker
A Circuit Breaker using a vacuum as the insulting medium.
4
1
INTRODUCTION AND SUMMARY OF FINDINGS
This report presents the findings of a technical investigation undertaken by the Authority
for Electricity Regulation, Oman into the incident that occurred at Muttrah Old Stores
Primary Substation on Tuesday 28 March 2006 and which resulted in fatal injuries to two
persons.
In the course of the investigation the inquiry met with key personnel of Muscat Electricity
Distribution Company SAOC (MEDC), Al-Wadi Contracting Company (Al-Wadi) and ABB
Arab, the contractors and suppliers involved, the Royal Omani Police and the Public
Prosecutor’s Office. The inquiry team also visited the site of the incident and reviewed
photographic evidence taken by Authority staff and other parties.
The analysis and conclusions presented in this report have been produced in response
to the information made available to the inquiry by all the persons and companies
involved in the incident. Throughout the investigation all persons and companies
involved co-operated fully with the investigation and the inquiry team wishes to express
its appreciation of the professional manner in which the recipients handled its inquiries,
which were sometimes of a sensitive and difficult nature.
1.1
Summary of Findings
The inquiry concludes that the most probable cause of the incident was a flashover
between the conductor arms on the circuit breaker truck manufactured by ABB Arab and
the rear feeder shutter on the original Brush cubicle. The evidence presented to the
inquiry attributes this flashover to shortcomings in the design by ABB Arab of the
interface between the truck and the cubicle.
With respect to contractual issues, the inquiry finds that Al-Wadi Contracting Company
was not qualified to bid for the Muttrah retrofitting contract, and that therefore Al-Wadi
Contracting Company should not have been awarded the contract. Furthermore, having
been awarded the contract Al-Wadi Contracting Company failed to ensure that the
5
relevant terms and conditions of their contract with MEDC were passed on to their
supplier ABB Arab.
On site safety the inquiry concludes that the use of a Limitation of Access for carrying
out the work was not appropriate given that the VCB/trucks had never been tested in a
Brush cubicle. However the design defects identified in this report would have
manifested themselves in any case irrespective of the site safety precautions taken
1.2
Recommendations
The inquiry makes a number of recommendations in respect of the circumstances
surrounding this incident:
Retrofitting
Further retrofitting of switchgear should not be carried out unless:
• It is carried out by the original equipment manufacturer (OEM); or
• If carried out by someone other than the OEM it shall be subject to testing in the
original cubicle and witnessed by a reputable switchgear test house.
Contracts
Licencees should ensure that:
• When submitting offers, contractors/suppliers comply strictly with all the terms and
conditions of the invitation to tender;
• That all the terms and conditions of a contract (e.g. production of drawings,
certificates etc ) are enforced;
• That close liaison is maintained between the Contracts Department and the field
staff responsible for the supervision of the execution of a contract.
Site Safety
Licencees should ensure that:
• A safety risk assessment is carried out for all non-routine work on or adjacent to live
equipment. This assessment should be carried out by an HSE specialist not involved
in the proposed work.
6
2
SCOPE OF INVESTIGATION AND METHODOLOGY
2.1
Summary of the Incident
At 1846 hrs on Tuesday 28 March 2006 on the distribution system of Muscat Electricity
Distribution Company at Muttrah Old Stores Primary Substation a flashover occurred on
the Busbar and Feeder spouts of the Al-Harthy/Local Transformer 11 KV Circuit Breaker.
As a result of the incident two employees of ABB Egypt, sub-contractors to Al-Wadi
Company were severely burnt and subsequently succumbed to their injuries in Khoula
Hospital on Sunday 2 April 2006.
2.2
Scope of investigation
As a result of the incident the Authority for Electricity Regulation, Oman (the ‘Authority’)
set up a technical inquiry with the following Terms of Reference:
To enquire into the technical circumstances surrounding the fatal accident that occurred
at Old Muttrah Stores Primary Substation at 1846 hrs on Tuesday 28 March 2005.
The inquiry shall specifically include the following:
a) The sequence of events that led up to the incident;
b) The safety precautions in place at the time of the incident;
c) The sequence of events following the incident including the provision of first aid to
the injured persons;
d) The specification and scope of the contract for retrofitting of vacuum switchgear;
e) The training given to all persons involved in the incident.
7
The inquiry shall be conducted pursuant to the powers granted to the Authority under
Article (22) para (3) of the Sector Law promulgated by Royal Decree No 78/2004 and
shall require any information to be provided to it.
The inquiry shall determine the causes of the incident and shall make recommendations
to the Authority for any changes in safety procedures and working practices it considers
necessary.
Following the incident the Public Prosecutor’s Office agreed that the Authority’s inquiry
would provide the technical input into its own investigations.
8
2.3
Methodology
In preparing its report the inquiry decided to concentrate on the following aspects:
a) The exact sequence of events leading up to the incident including the safety
precautions in place;
b) The reason for the flashover;
c) The actions taken subsequent to the incident;
d) The contract for the retrofitting of the Vacuum Circuit Breakers and the quality
assurance aspects of the contract;
e) The experience of the contractor concerned and experience of similar work carried
out in Oman; and
f)
The immediate remedial actions required
The inquiry sought independent technical advice from KEMA International and from
Sultan Qaboos University to ensure a rigorous and thorough evaluation of technical
matters. The inquiry also received full cooperation from the ABB Group including the
results of an internal technical review carried out by ABB Italy.
9
3
BACKGROUND – RETROFITTING OF SWITCHGEAR
Many thousands of bulk-oil circuit breakers (OCB) have been in service since the early
1960s and formed an integral part of most distribution substations at the 33 KV and 11
KV levels. Many of these circuit breakers are approaching or have exceeded twenty-five
years operational service life. In general, oil-filled switchgear has a proven record of
reliability and performance. Failures are rare but, where they occur, the results may be
catastrophic. Tanks may rupture, resulting in the ejection of burning oil and gas clouds,
causing death or serious injury to persons and major damage to plant and buildings in
the vicinity of the failed equipment.
Within the Medium Voltage (MV) switchgear sector, there is now increasing interest in
the options for the economic replacement of ageing oil based switchgear with switchgear
that utilises non-flammable substances (Vacuum or SF6) as the insulating and breaking
medium.
A typical MV switchboard consists of two parts;
(i)
A fixed portion (‘the cubicle’) containing the busbars, feeder terminations,
protection equipment, voltage and current transformers etc. The cubicle also
contains the safety shutters which are operated by the action of racking in the
moveable portion.
(ii)
A moveable or withdrawable portion (‘the truck or carriage’) which contains the
circuit breaker itself together with facilities for connecting the circuit breaker to the
busbars or feeder circuits.
For the replacement of time-expired switchgear there are two options available:
a) Replace the whole switchboard with one of modern design. This involves the
replacement of both the fixed and moveable portions of the switchboard by modern
equipment. This option is expensive in terms of capital cost: it also requires extensive
10
outages on the distribution system in order to connect the existing feeders to the new
switchboard; or
b) Replace only the moveable portion of the switchboard (‘the truck’) containing the new
circuit breaker. Only minor modifications are normally required to the fixed portion.
This second option is customarily known in the switchgear industry as ‘retrofitting’.
11
4
4.1
RETROFITTING AT MUTTRAH OLD STORES PRIMARY SUBSTATION
Substation Design and Layout
The 11 KV switchboard at Muttrah Old Stores Primary Substation is of Brush
manufacture and was installed in the late 1970s. The switchboard consists of two
transformer bays fed by 16 MVA transformers from the 33 KV system, a bus coupler bay
and eight feeder bays i.e. it consists of 11 cubicles coupled together. The layout of the
11 KV switchboard is shown in the diagram and photographs below.
FIGURE 1 Muttrah Primary Substation Layout
12
FIGURE 2
Brush OCB Truck
removed
Muttrah Old Stores 11 KV
Switchboard with the Bus
Coupler OCB truck removed
from its cubicle.
FIGURE 3
Brush OCB Truck
inserted
Muttrah Old Stores 11 KV Switchboard
showing the Al-Harthy feeder cubicle with
the original Brush OCB truck in the
inserted position.
13
4.2
The Contract for Retrofitting at Muttrah
In September 2004 the Ministry of Housing Electricity and Water (MHEW) issued an
invitation to tender for replacement of the Brush OCBs and trucks at Muttrah Old Stores
Primary Substation by modern Vacuum Circuit Breakers (VCB) or SF6 Circuit Breakers
(SCB) on new trucks.
Similar invitations to tender were also issued in respect of Al-Falaj and Al-Wattaya
Substations, which are also of Brush design and with the identical number of cubicles as
Muttrah.
Each invitation to tender called for the supply and installation of 11 VCBs or SCBs with
their accessories i.e. a total of 33 Circuit Breakers and trucks. For each substation the
contract called for the supply of circuit breakers with different current ratings for the
isolating contacts:
8 trucks with isolating contacts rated at 400 Amps for the feeder circuits
1 truck with isolating contacts rated at 800 Amps for the Bus Coupler circuit
2 trucks with isolating contacts rated at 1200 Amps for the Transformer circuit.
The invitation to tender required the tenderer to be a Class D authorised contractor
under the MHEW approved contractor procedure. Tender documents were sold by
MHEW to Al-Wadi Contracting Company despite Al-Wadi being a Class B/C registered
contractor.
Three offers were subsequently received by MHEW:
•
Al-Wadi Contracting Company,
•
Jyoti Sohar and
•
B.E.C.
Al-Wadi Contracting Company proposed to supply retrofit VCB trucks of ABB Arab
manufacture and offered the lowest price. The offers were evaluated and the three
14
contracts were subsequently awarded to Al-Wadi Contracting Company despite that
company not being qualified to bid since it was not a Class D contractor.
Al-Wadi Contracting Company then proceeded to sign two purchase orders with ABB
Arab in Egypt, one for the manufacture and supply of the 33 VCB/ trucks and the other
for the installation and commissioning.
Although the contract was awarded to Al-Wadi Company in late 2004, the Agreement
was not formally signed by Muscat Electricity Distribution Company (Successor
Company to MHEW) and Al-Wadi until 13 September 2005.
Between the award of the Contract and the signing of the Agreement, a sample Brush
OCB and truck was provided via Al Wadi to ABB Arab. ABB Arab then proceeded with
the manufacture and installation in about April 2005 of a ‘prototype’ VCB truck in the Bus
Coupler cubicle at Al Falaj substation. It has not been possible to find any paperwork
relating to these events.
In January 2006 the 33 VCB/truck assemblies were inspected in Egypt by engineers
from MEDC. In a letter to the Distribution Manager of MEDC the engineers reported:
1. That it was not possible to check the VCB/truck assemblies in a switchgear
cubicle since no such cubicle was available in Egypt and that only mechanical
and insulation tests on the truck assembly were carried out;
2. That the VCBs themselves had been type-tested in ABB Italy and the Test
Certificates had been provided; and
3. That the VCB/truck assemblies had been provided without the female isolating
contacts (‘tulips’). In this context the MEDC engineers suggested ‘informing ABB
and his agency that it’s not allowed to use any part of the existing Brush circuit
breaker and fixed it in the new VCB….’
The VCB/truck assemblies for the three Brush type substations were delivered during
March 2006. It should be noted that all 33 VCB/truck assemblies were delivered with 800
15
Amp isolating contacts. The contracts called for the delivery of 24 VCB/truck assemblies
with 400 Amp contacts, 3 with 800 Amp contacts and 6 with 1200 Amp contacts (see
above).
MEDC decided that the retrofitting should commence at Muttrah Old Stores Primary
Substation. The following section describes events leading up to the incident. It should
be noted that employees of ABB Arab (ABB) in accordance with the contract between
Al-Wadi and ABB carried out the actual installation work.
16
5
SEQUENCE OF EVENTS
The sequence is based on the correlated evidence of the investigation reports issued by
the Muscat Electricity Distribution Company (MEDC) and interviews with personnel
involved.
•
ABB commenced work on site on 26 March 2006. On 26 & 27 March no work was
carried out on the switchboard itself. The contractor was involved in unpacking and
inspecting the switchgear trucks (including the new vacuum circuit breakers).
•
At 0840 hrs on 28 March 2006 Limitation of Access (LOA) No 32152 was issued by a
Senior Authorised Person of MEDC to the supervisor of ABB. The LOA clearly
specifies that the 11KV Cables and Busbars were live. At the time of issue of the
LOA both Sections of the 11 KV Busbar were alive fed from their respective 33/11
KV Transformers. The following feeder circuit breakers were open but the 11 KV
cables were alive because of back feeding: Darsait, Oman House (on Section 1) and
Gharifa, Al Harthy (on Section 2)
•
The work to be carried out as specified on the LOA was:
To replace oil CBs to vacuum CBs for the following CBs Oman House, Ghanifa Fdr
and Darsait Fdr. Al Harhty Fdr
•
MEDC state that the LOA was issued to the contractor only for working on the Circuit
Breaker in the withdrawn position for doing the necessary testing and alignments.
MEDC also state that verbal instructions were given to the contractor to inform the
MEDC engineer, after completing his testing and alignments, that he was ready to
rack in the circuit breakers.
•
During the testing and alignment of the new circuit breaker trucks in the withdrawn
position ABB determined that the contacts (tulips) on the truck isolating contacts (i.e.
the contacts that engage in the busbar and feeder spouts when the breaker is racked
17
into the service position) were rated at 800 Amps and not 400 Amps as specified in
the contract. ABB decided to remove the 400 Amp contacts from the old Brush OCB
trucks and use them to replace the 800 Amp contacts on the new VCB trucks. In
order to do this it was necessary to machine a total of 100 copper terminals and ABB
staff proceeded to Wadi Khabeer Industrial Area in order to have this machining
carried out. The use of the tulip contacts from the original Brush trucks and the
machining of the copper terminals was not discussed with MEDC and therefore was
done without approval.
•
MEDC engineers left site at approximately 1500 hrs.
•
Subsequently ABB advised that they proceeded to rack in and rack out new VCB
trucks on 3 Feeders: Darsait, Oman House, and Gharifa. The VCB truck on the
Darsait feeder was left racked in and in the service position.
•
ABB also advise that the VCB truck associated with the Al Harthy feeder was then
racked in to the service position with the cubicle door open. The VCB itself was in the
open position with its protection inoperative. ABB were preparing to leave site and
were tidying up their tools when at 1846 hrs a flashover occurred inside the Al Harthy
feeder cubicle.
•
Two ABB employees were directly in front of the cubicle and received the full force of
the flashover, which ignited their clothing. They ran out of the substation and the
ABB supervisor assisted by members of the public extinguished the fire on their
clothing.
•
The ABB supervisor called the emergency services but there was a delay of some 45
minutes before the arrival of the ambulance. During this period it is understood that
the injured persons were not given any first aid. Following arrival of the ambulance
the injured persons were taken to Khoula Hospital where they subsequently
succumbed to their injuries.
18
The protection relay indications for the flashover (determined subsequently) were:
At Muttrah Stores Primary Substation:
On 16 MVA Transformer 2: Overcurrent, Earth Fault (Red, Blue – Earth)
Both 33 KV and 11 KV Circuit Breakers tripped.
On Al Harthy Feeder: Overcurrent, Earth Fault (Red, Blue – Earth). Auto Reclose
Block. Feeder Circuit Breaker Open.
At Darsait Primary Substation
On Muttrah Stores Feeder: Overcurrent, Earth Fault (Red, Blue – Earth), Auto
Reclose Block. 11 KV Circuit Breaker tripped.
19
6
TECHNICAL INVESTIGATION
Following the incident a series of investigations have taken place in order to determine
the probable cause of the incident. These investigations, led by the Authority have
involved personnel from MEDC, Al-Wadi and ABB. The Power Systems Engineering
Department of Sultan Qaboos University also assisted the inquiry. The Authority
engaged the services of KEMA to act as an independent expert and received a copy of
the internal Design Evaluation Report from ABB Italy.
The main basis of the technical findings was site inspections carried out by KEMA and
ABB on the failed equipment at Muttrah substation, on the ‘prototype’ equipment at AlFalaj substation and on the Brush OCB equipment remaining in service. In addition the
‘prototype’ VCB truck, a sample of the final delivered VCB truck and a Brush OCB truck
were taken to ABB Italy for the ABB Design Evaluation.
6.1
The KEMA Report
The KEMA report is reproduced in full at Appendix A.
The Report identifies the following damage to the failed VCB truck and Al Harthy cubicle
by arcing:
•
4 out of 6 isolating contacts
•
three bushings on the feeder side
•
three insulated copper conductors on the feeder side
•
one shutter on the feeder side
•
copper conductors near three top terminals of vacuum circuit breaker
•
copper conductors near two lower terminals of vacuum circuit breaker
•
cast resin housing of the vacuum interrupters
•
marks of welding on the earth contact on the truck caused by earth fault current
Photographs 3 and 4 show the simulated configuration of the VCB, cable side bushings
and one shutter on cable side.
20
BUSHING
SHUTTER
FIGURE 4
Reconstruction
of configuration
on cable side
COPPER
CONDUCTORS
FIGURE 5
Lower part of
truck with
VCB
VCB
FEEDER
SIDE
TERMINAL
OF VCB
The KEMA report puts forward three hypotheses for the cause of the flashover:
1. Hypothesis 1: Initial fault caused by isolating contacts;
2. Hypothesis 2: Initial fault caused by flashover to open shutter due to inadequate
design; and
3. Hypothesis 3: Initial fault caused by flashover to closing shutter.
21
As indicated in the KEMA Report it is often not possible in cases of this kind to be
definitive about the causes of the initial fault. On the balance of probabilities KEMA
conclude that Hypothesis 2 is the most likely cause of the fault.
The basis of Hypothesis 2 is that the ABB design of the copper conductors connecting
the isolating contacts to the VCB itself is such that inside the bushings the original
coaxial configuration of the Brush OCB trucks has been replaced by an asymmetrical
configuration, which reduces the air gap between the plastic sleeve of conductor and the
inner side of the bushing to a minimum. The distance of live parts to the shutter has also
been decreased considerably. Diagram 2 below shows a simplified representation of the
decreased air-gap between the shutter/bushing and the isolating contact arms. Tracks of
scratching and even cutting of this contact arm sleeve by the bushing have been found
on other retrofitted breakers of the same type after they had been racked in and out (see
Photograph 5 below).
FIGURE 6
Insulating Sleeve cut by
bushing.
This VCB truck had been
racked in and out of the
service position.
22
FIGURE 7 Truck – Cubicle Interface
This diagram is purely illustrative and is not to scale.
BRUSH OCB TRUCK
The shutter and bushing are
located on the cubicle (fixed).
The contacts and contact
conductor are part of the truck.
Clearance between
shutter & contacts
Shutter
Contact
Bushing
ABB VCB TRUCK
23
Note the decreased clearance between the bushing and the contact conductor on the
ABB VCB truck.
The KEMA Report also raises a number of other issues concerning the design of the
retrofit VCB trucks. These other design shortcomings are not thought to be a contributing
cause to the initial fault:
• The construction of the cylindrical part of the moving isolating contact fingers (‘tulip’)
differs from the original Brush design: it is of a different shape (not conical as in the
original) and tightened with non-original bolts (metric instead of imperial thread) and
additional washers. These were the parts machined at Wadi Khabir (see Sequence
of Events above). It was also noted that the original (800 Amp) cylinders were silver
plated: this plating has been removed by the machining. The holes in the copper
conductors for mounting the cylindrical part of the isolating contacts are oversized,
probably for adjustment purposes. This may affect the contact resistance and thus
the current withstand capability of the isolating contacts.
•
The design and construction of the retrofit breaker, supplied as a prototype in 2005
and installed in the Bus Coupler bay at Al-Falaj, differs in several material respects
from those delivered in 2006 e.g.: different dimensions of isolating contacts (400 A
and 800 A), steel truck, VCB, isolator discs, conductors and height of the copper
conductors.
•
During the site inspection a new retrofit VCB could not be inserted into the cubicle
because the truck appeared to be stuck against vital parts of the shutter operating
mechanism; pushing further would have damaged this latching mechanism. See
Photograph 7 below.
•
An attempt was made to rack in the failed VCB truck in the Al-Harthy feeder bay. The
construction appeared to be very critical, because the gap between the truck and
linkage of the shutter mechanism was less than 1 mm. Racking in with force and
without careful positioning could have resulted in damage to the operating
mechanism of the shutters. In addition the side plate on which the bracket to activate
24
the shutter mechanism is mounted is much weaker than on the original OCB truck,
which makes the operation of the shutters less reliable.
•
After the incident all the retrofit VCB trucks were racked out and replaced by the
original OCBs. After withdrawal of the breaker truck in the Oman House feeder at
Muttrah by engineers of MDEC a broken pin of the latching mechanism of one
shutter was found.
FIGURE 8
Interference with
shutter mechanism.
New VCB Truck
6.2
Failed VCB Truck
The ABB Italy Report
The inquiry was given access to an internal Report prepared by ABB Italy. Since the
report was internal to ABB it is not possible to reproduce it here. However the inquiry is
able to summarise the broad findings of the ABB Report.
The ABB Italy report identifies the following design deficiencies:
Contact arms
a) The contact arms are not symmetrical in respect of the center of the bushing.
Considering the bending tolerances the contact arms may have interference with the
internal diameter of the bushing.
b) The contact arms are made by a copper bar of rectangular cross section 10x50mm
with the edge radius of 2mm. Such edge radius is not ideal for the heat-shrink insulation,
increasing the possibility of damage of the insulation during the assembling, packaging
25
or the transportation. Damage of insulation may also occur due to the interference of the
edge of the bar with the internal diameter of the bushing (see point a).
Support of the tulip contacts
The design of the support of the tulip contacts is correct, as it retains the same shape as
in the original circuit-breaker, but the supports assembled on site (400A contact system)
are not according to the drawing (800A contact system). The assembled shape tends to
reduce the movement of the tulip contacts in comparison with the original solution.
Stability of the truck
The stiffness in the longitudinal direction of the truck of the retrofit solution is comparable
with the original one. Not so for the stiffness in the transverse direction: here the
structure is quite flexible while the original truck is more robust. The transverse flexibility
could affect the correct position of the system to operate the shutter, particularly in the
case where the panel floor was not so well leveled.
The design deficiencies noted above are the same as those identified in the KEMA
Report.
Circuit-Breaker locating device
In addition to the above, ABB Italy identified a further deficiency relating to the circuit
breaker truck locating device. This device locks the truck in the panel in one of three
positions test, service, and earth.
On the original Brush OCB trucks this device is
interlocked to prevent the circuit breaker being racked in when the truck is not in the
correct position; in addition on the Brush OCB truck it is not possible to unlock or move
the truck with the circuit breaker racked in. This interlocking arrangement was not
present on the trucks supplied by ABB Arab. If the operation to lift the circuit-breaker
starts in an incorrect position the shutter could be damaged or not correctly opened.
26
6.3
Cause of the Incident
Based on the above findings from the site inspections and evaluations by both KEMA
and ABB Italy it is possible to postulate the probable failure mode.
FIGURE 9 Hypothesis of the Failure Mode
After the VCB/truck was racked into the
service position arcing commenced
between the contact arm, the bushing,
the shutter and the top plate of the
cubicle on one of the outer phases on
the feeder side.. This established a
phase-to-earth fault.
Following ionisation of the air a second
earth fault was established across the
bushing of one of the other phases on
the feeder side.
With two earth faults on the feeder side
a two phase short circuit was thus
established. This short circuit arc would
be fed by the feeder current and was
forced downwards (i.e. away from the
source) by the electromagnetic forces
until it jumped to the lower terminals of
the VCB itself.
Extreme ionization now exists in the
area around the six copper conductor
arms. A three phase fault then occurs
on the top terminals of the VCB. This
fault is fed from the busbar with much
higher power and taking at least 1
second to clear on the back-up
protection.
27
FIGURE 10 Contact Damage
The damage found on the isolating
contacts (‘tulips’) is probably due to
improper assembling and design and
thus could not withstand the short circuit
current from the feeder and busbar.
6.4
Technical Conclusions
In an investigation of this type it is not possible to be definitive about the exact cause of
failure. However both the KEMA and ABB Italy Reports identify the most probable cause
as a flashover between the conductor arms on the truck and the rear feeder shutter on
the cubicle.
Both reports are coincident in attributing this failure to shortcomings in the design by
ABB Arab of the interface between the truck and the cubicle. Furthermore as indicated in
the report by MEDC engineers, the truck assembly was never tested in a Brush cubicle
which could have identified the design shortcomings prior to installation.
Additional shortcomings in the design and manufacturing of the truck assembly were
also identified in both reports. These shortcomings include:
• Contact support pillars and their machining on-site;
• Low transversal stiffness of the truck which could have affected operation of the
shutter mechanism;
• Inadequate design of the truck position interlocking mechanism.
On the basis of the above findings the inquiry concludes that the prime cause of the
failure at Muttrah was poor design by ABB Arab of the interface between the truck and
cubicle. Furthermore there were a number of other design and manufacturing
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shortcomings in the trucks supplied which calls into question the quality control
procedures of ABB Arab.
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7
THE CONTRACT
7.1
Placing of the Contract
As indicated in Section 4.2 above the contract for retrofitting at Muttrah was placed as a
result of an Invitation to Tender issued by the Ministry of Housing, Electricity & Water in
September 2004. Al Wadi Contracting Company submitted their offer on 24 October
2004.
As noted above Al Wadi Contracting Company was not qualified to bid for the contract
as it was a registered Class B/C contractor whereas the Invitation to Tender called for a
Class D contractor. Al Wadi proposed to supply new VCB/trucks from ABB Arab and
indicated so in their tender.
The three bids received as a result of the Invitation to Tender were passed by the
Tenders & Contracts Department of MHEW to the Transmission & Control Department
for technical evaluation. To date the inquiry has not received a copy of the evaluation
carried out by the Transmission & Control Department nor of the award of the Contract
to Al Wadi. Indeed it is unclear whether the Contract was awarded by MHEW or was
awarded by MEDC in their letter of 13 September 2005.
The terms of the ‘sub-contract’ between Al Wadi and ABB Arab are unclear as both ABB
Arab and Al Wadi were unwilling to release details to the inquiry for commercial reasons.
ABB Arab has indicated that the agreement between them was in the form of a simple
purchase order and the inquiry concludes that the relevant terms and conditions of the
contract between MEDC and Al Wadi were not passed on to ABB Arab.
In the period between the submission of the offer (October 2004) and the ‘award’ of
Contract (September 2005) MHEW provided a Brush OCB truck to Al Wadi. This
enabled ABB Arab to design and manufacture a prototype VCB/truck and to install it in
the bus-coupler bay at Al Falaj substation around April 2005. Again the inquiry has been
unable to obtain any written evidence of what took place in this period.
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Subsequent to the incident it emerged that Al Wadi did not hold any insurance for the
Contract even though it was an express requirement of the Contract. Discussions with
MEDC have indicated that it was not the practice to require production of Insurance
Certificates until the contractor requested the Advance Payment (normally 10%) under
the contract. This practice has now been changed and contractors are required to
produce such certificates on signing the contract.
7.2
The Contract – Conclusions
Based on it’s analysis of the documentation available the inquiry concludes the following:
• Al Wadi Contracting Company was not qualified to bid for the Muttrah retrofitting
contract;
• Al Wadi should not have been awarded the contract;
• Al Wadi did not ensure that the relevant terms and conditions of their contract with
MEDC were passed on to ABB Arab;
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8
8.1
SITE SAFETY
Use of Limitation of Access
As indicated in Section 6 above the work at Muttrah substation was being carried out
under a Limitation of Access issued by a duly authorised engineer of MEDC. According
to the Safety Rules of the MHEW (the Rules currently in force in MEDC) a Limitation of
Access is a form issued and cancelled by a Senior Authorised Person or Authorised
Person specifically authorised to do so, defining the limits and nature of work which may
be carried out in the vicinity of Live apparatus.
Under normal circumstances the use of a Limitation of Access (rather than a Permit to
Work or Sanction for Test) for the work being carried out at Muttrah was perfectly
acceptable. The racking in/out of OCB/trucks with the busbar and feeder alive is
common practice and does not normally require even a Limitation of Access. However in
the particular circumstances of the incident the VCB/trucks were actually being adjusted
to fit and tested in a Brush cubicle for the first time. The use of a Permit to Work (with the
busbar and feeder spouts dead) would have been more appropriate. It should be noted
that the incident could still have occurred once the busbar and feeders were made alive
following cancellation of the Permit to Work because of the design defects noted in this
report.
In their report on the incident MEDC state that verbal instructions were given to the ABB
Arab technicians that they were to carry out all adjustments of the trucks in the rackedout position and that they should inform MEDC when they were ready to rack-in the
breakers. ABB Arab has disputed that these instructions were ever given. Any additional
instructions to be given in respect of a Limitation of Access, for example, should be
written on the Limitation. Having said that, the inquiry believes that the presence or not
of MEDC engineers is irrelevant; the design defects would have manifested themselves
whenever the breaker was racked into the service position.
8.2 Conclusions – Site Safety
In respect of site safety precautions the inquiry concludes:
• That the use of a Limitation of Access was not appropriate given that the VCB/trucks
had never been tested in a Brush cubicle; and
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• That the design defects identified in this report would have manifested themselves in
any case irrespective of the site safety precautions.
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9
FIRST AID
From the information available to the inquiry, no first aid was given to the injured persons
in the immediate aftermath of the incident. The inquiry notes that some 45 minutes
elapsed before the arrival of the ambulance but does not feel competent to determine
whether this delay had any effect on the ultimate outcome.
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10 RECOMMENDATIONS
The inquiry makes the following recommendations:
10.1 Retrofitting
Further retrofitting of switchgear should not be carried out unless:
• It is carried out by the original equipment manufacturer (OEM); or
• If carried out by someone other than the OEM it shall be subject to testing in the
original cubicle and witnessed by a reputable switchgear test house.
10.2 Contracts
Licencees should ensure that:
• When submitting offers, contractors/suppliers comply strictly with all the terms and
conditions of the invitation to tender and that they are qualified to submit a tender;
• That all the terms and conditions of a contract (e.g. production of drawings,
certificates etc ) are enforced;
• That close liaison is maintained between the Contracts Department and the field
staff responsible for the supervision of the execution of a contract.
10.3 Site Safety
Licencees should ensure that:
• A safety risk assessment is carried out for all non-routine work on or adjacent to live
equipment. This assessment should be carried out by an HSE specialist not involved
in the proposed work.
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