|
Anne
McIntosh , EA Technology's materials and failure
investigations lead consultant, explains how
forensic science is improving asset reliability
and safety.
UNDERSTANDING FAILURE IS THE
KEY TO ASSET SAFETY
Failures of medium and high
voltage electrical assets often make headline
news, such as the recent underground cable failure
which resulted in 20,000 Liverpool homes being
without power, or the transformer failure that
left 250,000 Madrid residents in darkness. Similar
outages have hit New York, London and Birmingham
in recent months. So why do incidents like these
happen?
Forensic science is best known
for identifying the causes of aircraft crashes
or gathering criminal evidence. But it is increasingly
recognised as the key to enhancing the safety
and reliability of electrical assets, by enabling
us to understand the sequence of events leading
to failure.
|
 |
Just as investigations into failures have dramatically
improved aircraft design and operation, and made air
travel much safer, expert examination of failed assets
is vital to improving equipment design and maintenance
procedures. It also enables operators to make informed
decisions on asset management, including investment
and replacement. Lessons learnt from failure investigations
will reduce the occurrence of expensive and dangerous
failures and ultimately lead to an increase in the safety
and reliability of assets.
In many instances, forensic investigations are needed
to provide evidence of liability in legal cases, where
there may be safety as well as financial implications.
Failures which were once regarded as unfortunate accidents
are increasingly classed as preventable incidents, for
which somebody will have to bear the responsibility.
Failures of MV and HV electrical equipment such as
switchgear, transformers, cables and overhead lines
are often sudden and catastrophic, causing considerable
collateral damage to other equipment and sometimes resulting
in injury and death. Operators therefore need to be
prepared for the possibility of prosecution under Health
and Safety regulations, as well as apportioning financial
responsibility. Charges may even include corporate manslaughter.
Our main roles as specialists in forensic investigation
are to determine the primary cause of each failure and
present recommendations for preventing similar occurrences
in future.
Failures are usually caused by one or more factors,
including design, materials selection, fabrication and
processing, operating and service conditions, human
error, vandalism and illegal activity. For example,
meticulous examination following a catastrophic failure
of an oil filled circuit breaker identified a polyvinylchloride
(PVC) ring within the dashpot. Over a period of time,
the plasticizers within the PVC leached out into the
oil, causing the ring to shrink and harden. This resulted
in incorrect operation of the circuit breaker and arcing
between the slightly opened male and female contacts.
Further arcing occurred, generating explosive gases,
which blew the tank lid off. It was recommended that
the PVC ring be replaced by a high grade nitrile rubber,
which is resilient to oil.
Initially, a failure investigation should be directed
to collecting all the relevant background and historical
information. This information should include reconstruction
of the events leading to the failure, manufacturing
histories, service records, operational conditions and
details of any repairs, maintenance or modifications.
This information can be of great help in understanding
the failure mechanism.
Making sense of the often charred and twisted results
of an equipment failure relies on two approaches, used
in tandem: sophisticated laboratory techniques, plus
expertise in interpreting evidence and patterns of events,
based on nearly 40 years of similar investigations.
Our collective know-how is supported by a database which
contains records of how thousands of assets have deteriorated
and failed in the past.
In the laboratory, we can perform comprehensive testing
of equipment, including electrical testing. Many materials
and chemical analysis techniques can be performed on
materials used in switchgear and transformers, from
metals to plastics. Low powered optical microscopes
are used to examine the surfaces of samples, whilst
higher powered microscopy with magnification up to x1250
is used in conjunction with image analysers to examine
and identify fine particles and fibres.
Scanning electron microscopy raises magnification
as high as x100,000, enabling us to carry out x-ray
elemental analysis quickly and accurately, while quantitative
chemical analysis is carried out on features as small
as 0.001m 3. Applications include the study of surface
morphology, microstructure and elemental compositions
of a wide range of materials.
Infra red spectrometry is used to identify the fingerprints
of organic samples, including plastics such as polyethylene,
used in cable insulation. We also use a variety of mechanical
testing equipment to measure the tensile, compression,
torsional and hardness of a wide range of materials.
Analytical results are only as good as the samples
provided. This means that, not only must samples be
retrieved and handled with the utmost care to avoid
damage and loss of evidence, but they must also be transported
correctly.
Lab tests produce a mass of very detailed information,
but the most important part of our work is the ability
to interpret such data, to present an accurate picture
of how and why the failure happened. The results are
then fed back to our clients in a confidential report,
as well as being added to our database and enhancing
even further our understanding of failure patterns.
Recommendations following a failure investigation
are aimed at ensuring similar failures do not occur
in the future. Depending on the failure mechanism, numerous
actions can be taken, including employing condition
assessment techniques in order to identify if the remaining
population of a failed asset are prone to failure. Useful
diagnostics include visual examination, partial discharge
monitoring, electrical testing and thermal imagery.
In some instances, changes in the design of equipment
or the materials used are required.
Adopting improved maintenance practices can combat
against failures occurring. Prevention is always better
than cure and increased understanding of the processes
leading to HV asset failure has certainly proved essential
in our development of new maintenance techniques, particularly
for oil filled equipment.
Engineers are clearly concerned
about their need to get up to speed on the safety of
HV assets, which has led us to extend our series of
Keeping Electrical Switchgear Safe conferences to 11
in the last 18 months, with more to come in future.
The events focus on issues raised by the Health & Safety
Executive's recent report on best practice in switchgear
safety, and all have been sell-outs so far.
They include presentations on the latest legal requirements
for safe switchgear management and operation, guidelines
on required maintenance regimes, training and operational
protocols, and options for replacing or upgrading equipment.
As the HSE report points out, adopting best practice
on the management of switchgear safety will prevent
many failures. It will also provide a defence in prosecutions,
which are increasingly likely to result from failure
incidents.
Dedicated courses regarding failures of HV assets
and their implications are available.
Specialist forensic investigation is part of the
process of making failures less common, by identifying
preventable causes. But eliminating failures altogether
will take concerted effort by asset manufacturers, owners
and engineers over many years.
|
Failures are often sudden and catastrophic.
|
|
Failures are often sudden and catastrophic.
|
|
Failures typically cause widespread damage.
|
|
Switchgear failure.
|
|
EA Technology uses optical and electron scanning
microscopy.
|
|
Discharge on the surface of a contaminated porcelain
insulator.
|
|
Formation of filaments within a moving contact
assembly in a fused isolator, associated with contamination
of the silver plated contacts in a sulphur rich environment.
|
|
Alignment of fibres in oil between electrodes.
|
|
A microscope reveals opposite pitting on a copper
conductor.
|
|
