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www.netaworld.org Spring 2008 NETA WORLD
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Testing and Maintaining
Low-Voltage, Bolted PressureSwitches
by Lynn HamrickESCO Energy Services
It seems to happen during every outage. You are brought in to
maintain andtest medium-voltage electrical distribution equipment.
You are rolling along
with your scope o work and get a call rom the client to assist
in resolving
a problem with some low-voltage electrical distribution
equipment. Most o the
time, the problem is associated with the larger, used-disconnect
switches which
are being used as mains and eeders or the low-voltage electrical
inrastructure.
Tey either will not open or will not close. When you
investigate, you nd
maintenance people huddled around one o the ollowing: a Bolt-Loc
switch, a
Pringle switch, a Boltswitch, a BP Switch, or a GE HPC switch.
All o these
devices are commonly reerred to as bolted pressure switches and
are typicallyrated rom 800 to 4000 ampere. Te bolted pressure
switches o today are a
derivation o the open knie switch prevalent in the 1900s, with
improvements
having been made or sae and reliable operation. Most o these
switches are
manually operated, although they can be electrically tripped
with the addition
o shunt trip capability. Due to their size, they are typically
spring-assisted or
opening and closing.
Tese devices are ound in many industrial complexes, since they
are consideredto be cost efective solutions or the industrial
plants electrical inrastructureneeds or circuit protection and
electrical system isolation. Tese devices can
provide years o reliable service with appropriate preventive
maintenance. Unlikecircuit breakers, these devices rely on uses to
provide ault clearing capability;thereore, they do not require a
mechanical operation, such as the switch opening,to provide
associated circuit protection. Switch operation is typically
perormedin support o either service and maintenance activities or
use replacement toensure adequate electrical circuit isolation. Tis
circumstance can lead to longperiods o inactivity where the switchs
mechanical and electrical componentsnever move. Tereore, these
switches must be tested and maintained to ensureproper operation
when required.
Different MaintenanceStrategies
Tree basic maintenance strate-gies are typically applied to
boltedpressure switches:
Corrective Maintenance Withthis strategy, maintenance is
per-ormed as the switch ails to oper-ate. Although considered
short-sighted by most maintenanceproessionals, this is a
prevalentmaintenance philosophy in mostmedium to small industrials.
Jus-tiying the perormance o preven-tive maintenance is a
comparativeexercise which typically involvescomparing short-term
savings inmaintenance costs to potentialrepair costs and production
losses.
Te unortunate result in applyingthis maintenance strategy is
thatthere may be inoperable switchesin the electrical system. Te
ormo corrective maintenance can vary
greatly. A common methodologyor closing a switch that will
notclose is to incorporate a hammerand 2X4. Tis method o
correctivemaintenance is usually associated
with encouragement rom theproduction personnel. It is
notrecommended, however, due to thelong-term efects on the switch
inthat it probably will not open whenrequired and the contact
resistance
will not be optimal. Te correct
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NETA WORLD Spring 2008 www.netaworld.org
Copyright2007InterNational Electrical TestingAssociation 2700W.
CentreAvenue, SuiteA Portage, MI 49024Voice: 269.488.6382 Fax:
269.488.6383 Email: [email protected] Web: www.netaworld.org
MTS-2007 Appendix BFrequency ofMaintenance Tests
ANSI/NETA MTS-2007
American National Standard
InterNationalElectricalTesting Association
for Electrical Power DistributionEquipment and Systems
2007
methodology is to ollow the manuacturers recommenda-tions by
cleaning and lubricating the switch and adjustingthe closing
mechanism to accommodate proper closing othe switch.
Time Interval-Based Maintenance With this strategy,maintenance
activities are perormed at a predeterminedrequency regardless o the
conditions under which a switchoperates. I this method is applied
too strictly, however, itmay lead to excessive work eforts and
costs.
Condition-Based Maintenance With this strategy, thecondition o a
switch is evaluated through maintenancetesting and inspection. Te
results, supplemented withstatistical data and cumulative
experience, are then usedor maintenance planning. Condition-based
maintenanceprovides excellent opportunities to improve reliability
andcut costs, but it requires efective diagnostic methods. Dueto
the level o current typically associated with boltedpressure
switches, the perormance o an annual inrared(IR) survey o the
switch contacts and cable or bus connec-tions or these switches
should be considered mandatory
with any condition-based maintenance program.
Periodicinspections should include looking or signs o heat
discol-oration and contamination buildup on contact points
andconnections.
Periodic testing o the switchshould also be perormed.ANSI/NEA
MS-2007,Standard or Maintenance est-ing Specications or
ElectricalPower, Distribution Equipmentand Systems, Appendix B,
Fre-quency o Maintenance ests,could be used as a basis or
de-termining the initial inspection
and testing requency. ANSI/NEA MS-2007 recom-mends annual
visual/mechani-
cal inspections and testing perormed every three (3)
years.Recommended inspection and testing intervals should thenbe
rened urther as described in the MS-2007 appendixand also in NFPA
70B-2006, Recommended Practice orElectrical Equipment Maintenance.
Both standards giveguidance based on operating characteristics,
environmentalconditions, and maintenance history.
Suggested Electrical TestsAter installation, most bolted
pressure switches remain
in use until they are inoperable or are partially
destroyedduring an electrical event. For those switches that are
inservice, there are some basic electrical tests that should
beperormed:
Contact resistance Tis test should be perormed romline-to-load
terminals or the switch contact with the switchclosed. Te values
should be within 50 percent o each otheand comparable to similar
switches. Resistance values o lesthan 100 microhms are typical with
some manuacturerssuggesting values o less than 30 microhms.
Insulation resistance Tis test should be perormedphase-to-phase
and phase-to-ground with the switch closedAcceptable values should
be greater than 100 megohmsor 600 volt ac switches and 25 megohms
or 240 volt acswitches.
Fuse resistance Tis test should be perormed acrossthe
line-to-load terminals o the use. Te values should be
within 15% o each other and comparable to similar usesResistance
values will vary greatly based on type and size o
the use; thereore, there is no typically expected value.Ground
Fault and Shunt Trip Operability Where ap-plicable, ground ault and
shunt trip circuitry should be
veried or operability.Experience with bolted pressure switches
suggests tha
most problems associated with switch operability are due tothe
environmental conditions, improper or lack o cleaningand improper
lubrication o the switch. Tereore, cleaningand operability
verication should be perormed on the sameinterval as the electrical
tests recommended above. Cleaningactivities should include complete
removal o existing lubri-cating greases and deposits rom moving
parts and contacsuraces. Cleaning switch suraces should not include
the
use o solvents which may leave residues on suraces or theuse o
abrasive cloths (i.e. emery cloths, etc.) due to thepotentially
harmul efects on the suraces. In most cases, theelectrical contact
suraces o these switches are plated, andusing abrasive materials to
clean the suraces may removeor damage that plating. Application o a
very thin layer onew lubricant should be provided in accordance
with themanuacturers recommended locations using
recommendedlubricants. I reiterate that one should apply a very
thin layeo lubricant, since too much grease, particularly on
thecontact suraces, can be detrimental to switch operability
Where no lubricant recommendations are specied by the
manuacturer, we typically use Mobilgrease 28 on both thecontact
suraces and the hinge points o the switch.Further, blade alignment,
blade penetration, and the
mechanical open and close operations should be veriedMost
manuacturers recommend opening and closing theseswitches at least
three times annually as part o the preven-tive maintenance
activities.
