ASTM E780-17(2021)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E780 − 17 (Reapproved 2021) An American National Standard
Standard Test Method for
Measuring the Insulation Resistance of Mineral-Insulated,
Metal-Sheathed Thermocouples and Mineral-Insulated,
Metal-Sheathed Cable at Room Temperature
This standard is issued under the fixed designation E780; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope and experience must be applied in establishing test voltage
levels and acceptable insulation resistance values for these
1.1 This test method provides the procedures for measuring
types of thermocouples and MIMS cables.
the room temperature electrical insulation resistance between
the thermoelements and between the thermoelements and the 1.3 This test method may be used for thermocouples or
sheath, of a mineral-insulated, metal-sheathed (MIMS) ther- MIMScableshavinganoutsidediameterof0.5mm(0.020in.)
mocouple or mineral-insulated, metal-sheathed (MIMS) ther- or larger.
mocouple cable or between the conductors and between the
1.4 Users of this test method should be aware that the room
conductors and the sheath, of mineral-insulated, metal-
temperatureinsulationresistanceofamineral-insulated,metal-
sheathed(MIMS)cableusedforindustrialresistancethermom-
sheathed thermocouple or MIMS cable will change during
eters. It may be used to measure the insulation resistance of
shipment, storage, or use if they are not properly sealed.
bulk lengths of mineral-insulated, metal-sheathed MIMS cable
1.5 The values stated in SI units are to be regarded as
previously sealed against moisture intrusion or to test a
standard. The values given in parentheses are for information
thermocouple having an ungrounded measuring junction. This
only.
method cannot be used to test a thermocouple having a
1.6 This standard does not purport to address all of the
grounded measuring junction unless the measuring junction is
removed prior to testing, after which the thermocouple may be safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
dealt with in the same manner as a mineral-insulated, metal-
sheathed (MIMS) cable. priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.2 This test method applies primarily to thermocouple
1.7 This international standard was developed in accor-
cables and cable used for industrial resistance thermometers
dance with internationally recognized principles on standard-
conforming to Specifications E585/E585M, E2181/E2181M,
ization established in the Decision on Principles for the
and E2821 and to thermocouples conforming to Specifications
Development of International Standards, Guides and Recom-
E608/E608M and E2181/E2181M, but may also be applied to
mendations issued by the World Trade Organization Technical
thermocouples or MIMS cables that are suitable for use in air,
Barriers to Trade (TBT) Committee.
whose sheath or thermoelements or conductors are comprised
of refractory metals, that are tested in a dry and chemically
2. Referenced Documents
inert environment, and that may employ compacted ceramic
2.1 ASTM Standards:
insulating materials other than magnesia (MgO) or alumina
E235 Specification for Type K and Type N Mineral-
(Al O ). Users of this test method should note that specifica-
2 3
Insulated, Metal-Sheathed Thermocouples for Nuclear or
tions dealing with compacted ceramic insulating materials
for Other High-Reliability Applications
other than magnesia or alumina, which are described in
E344Terminology Relating to Thermometry and Hydrom-
Specification E1652, are not currently available. As a result,
etry
acceptance criteria must be agreed upon between the customer
E585/E585M Specification for Compacted Mineral-
and supplier at the time of purchase, or alternatively, judgment
Insulated, Metal-Sheathed, Base Metal Thermocouple
Cable
This test method is under the jurisdiction of ASTM Committee E20 on
Temperature Measurement and is the direct responsibility of Subcommittee E20.14
on Thermocouples - Testing. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 1, 2021. Published November 2021. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1992. Last previous edition approved in 2017 as E780–17. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/E0780-17R21. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E780 − 17 (2021)
E608/E608MSpecification for Mineral-Insulated, Metal- out further preparation. If preparation of the MIMS cable is
Sheathed Base Metal Thermocouples required, special precautions may be necessary to prevent the
E1652Specification for Magnesium Oxide and Aluminum
intrusionofmoistureandothercontaminantsthatcanaffectthe
Oxide Powder and Crushable Insulators Used in the insulation resistance. The repeatability of the test method can
Manufacture of Base Metal Thermocouples, Metal-
primarily depend upon how well this is achieved. Preparation
Sheathed Platinum Resistance Thermometers, and Noble
usually involves removing 10 to 30 mm (0.4 to 1.2 in.) of the
Metal Thermocouples
sheath from each end of the MIMS cable, preventing the
E2181/E2181M Specification for Compacted Mineral-
intrusion of any moisture into, or expelling any moisture from,
Insulated, Metal-Sheathed, Noble Metal Thermocouples
the compacted mineral insulation, and sealing the ends with
and Thermocouple Cable
epoxyresinorothersuitablemoisturesealant.Usersofthistest
E2821 Specification for Compacted Mineral-Insulated,
method may refer to Appendix X1 for information.
Metal-SheathedCableUsedinIndustrialResistanceTher-
mometers
5. Significance and Use
5.1 Thermocouplesfabricatedfromthermocouplecablethat
3. Terminology
has been contaminated by moisture or by other impurities may
3.1 Definitions—ThedefinitionsgiveninTerminologyE344
undergo large changes in thermoelectric properties or may fail
shall apply to the terms used in this test method.
catastrophically when exposed to high temperatures. Since
3.2 Definitions of Terms Specific to This Standard:
such contamination usually lowers the electrical resistance
3.2.1 bulk material length (BML), n—a single length of
between the thermoelements and the sheath substantially,
finished thermocouple MIMS cable.
measurement of the insulation resistance can provide a valu-
able check of insulation quality and cleanliness, and can serve
3.2.2 dry, adj—refers to a condition of the ambient air at
timeoftestthatdoesnotexceedtheequivalentof50%relative as a basis for rejection of unsuitable material and unreliable
components. For manufacturers in particular, low electrical
humidity at 22°C [72°F].
insulation resistance can also be indicative of displaced ther-
3.2.3 thermocouple, n—refers to a mineral-insulated, metal-
moelementsorconductorsordefectsinthemetalsheathwhich
sheathed (MIMS) thermocouple.
will require further investigation, but all users should be aware
3.2.4 thermocouple cable, n—refers to a mineral-insulated,
of these potential defects when faced with an unacceptable
metal-sheathed (MIMS) thermocouple cable.
insulation resistance measurement.
4. Summary of Test Method
5.2 This test method is primarily intended for use by
manufacturers and users of mineral-insulated, metal-sheathed
4.1 This test method measures the room temperature (22 6
(MIMS) thermocouples or MIMS cables to verify that mea-
5°C (72 6 10°F)) dc electrical insulation resistance: (1) in the
suredvaluesofinsulationresistanceexceedspecifiedminimum
case of a length of MIMS cable, between each of the
values, such as those listed in Specifications E235, E585/
thermoelements or conductors and between the thermoele-
E585M, E608/E608M, E2181/E2181M, and E2821. Manufac-
ments or conductors and the sheath; (2) in the case of either a
turers and users should be aware, however, that when the
thermocouple having a single, ungrounded measuring junction
insulationresistanceisgreaterthan1×10 Ω,disagreementby
or a thermocouple having multiple thermoelement pairs which
an order of magnitude in the results obtained with this test
share a common, ungrounded measuring junction, between the
method is not unusual. In addition, users of this test method
thermoelement pair(s) and the sheath; (3) in the case of a
should appreciate that the room temperature insulation resis-
thermocouple having multiple, isolated, ungrounded measur-
tance of both MIMS cables and of finished thermocouples will
ing junctions, between each of the thermoelement pairs and
change during shipment, storage, and use if the end seals are
between the thermoelement pairs and the sheath. The resis-
damaged or defective. Consequently, values of insulation
tance measurements are made with an instrument such as a
resistance determined by this test method may not necessarily
megohm bridge or megohmeter as described in 6.2.
be repeatable.
4.2 In general, because removal of the measuring junction
would be necessary, measurement of the insulation resistance
6. Apparatus
between all thermoelements in a thermocouple is not com-
monly undertaken. Testing is limited to measuring the insula-
6.1 Warning,alltoolsandapparatususedmustbecleanand
tionresistancebetweenthethermoelementpairsandthesheath
must not introduce oil or other contaminants into the insula-
of the thermocouple and, where possible, the thermoelement
tion.Thepresenceofsuchcontaminantsmayinvalidatethetest
pairs.
results obtained using this test method.
4.3 Specialpreparationofathermocouplewillnotnormally
6.2 Megohmeter or Megohm Bridge, with a test voltage
be required, provided that the extension lead wires are clean,
range between 50 and 500 VDC, measurement ranges from
5 12
undamaged, and sufficiently long to permit connection of the
1×10 Ωto1×10 Ω, and an accuracy of at least 610% of
test instrument.
reading. Both the positive and negative connection terminals
andtestleadsaretobeelectrically“floating”(notconnectedto
4.4 AMIMScablehavingeffectiveendsealsinplaceandits
thermoelements or conductors accessible may be tested with- earth ground potential).
E780 − 17 (2021)
6.2.1 Other resistance-measuring instruments or circuits 8.1.2.1 Electrically connect all the thermoelements or con-
that satisfy the electrical requirements given in 6.2 are accept- ductors within the BML or all the thermoelement pairs within
able. the thermocouple by twisting them together or mechanically
short-circuitingthemattheendatwhichthetestvoltagewillbe
6.3 Insulated Copper Connecting Wires, with suitable
applied. Verify that no thermoelement, conductor, or thermo-
mechanical-type connectors.
element pair is in contact with the sheath at either end of the
6.4 Thefollowingapparatusmayberequiredincarryingout
cable or at the cold junction. The sheath should be electrically
the procedures described in Appendix X1:
connected to ground.
6.4.1 Heat Source,(forexample,asmallpropane-typetorch
8.1.2.2 Connect the positive lead of the measuring instru-
or an electric heat gun).
ment to the thermoelements or conductors or thermoelement
6.4.2 Moisture Sealant, such as epoxy resin , wax, or hot
pairs,thenegativeleadtothemetalsheath,recordthetime,and
melt glue that when properly applied will provide an effective
energize the test circuit.
seal against moisture intrusion for the end(s) of the thermo-
8.1.2.3 Select the lowest range of the measuring instrument
couple or MIMS cable at temperatures up to 66°C (150°F).
that will provide an on-scale reading.
Additional sealants, with higher temperature ratings, are avail-
8.1.2.4 Maintain the applied test voltage until the measured
able. The higher temperature sealants require additional pro-
valuestabilizesorforamaximumtimeof1minandrecordthe
cedures to ensure a proper seal.
reading indicated by the measuring instrument. De-energize
6.4.3 Metal-Sheathed Cable Stripper—Any commercially
the test circuit, making sure that any capacitively stored
available cable stripper that will satisfactorily remove the
electricchargehasbeendischarged.Donotdisconnectanytest
sheath without damage to the thermoelements or conductors is
lead wire from either the thermoelements or conductors (or
acceptable.
thermoelement pairs) or metal sheath without first de-
6.4.4 Optical Magnifier, with a magnification of 5 to 10×
energizing the measuring instrument’s test circuit.
(for example, a watchmaker’s loupe).
8.1.2.5 In the event of a BML or thermocouple (having
more than one isolated, ungrounded measuring junction) pro-
7. Test Specimen
viding an unacceptably low measurement value, individual
7.1 Conduct the insulation resistance measurements on the
measurements of the insulation resistance between each
full length of mineral-insulated, metal-sheathed (MIMS) cable
thermoelement, conductor, or thermoelement pair and the
or on the intact thermocouple sensor assembly under test.
sheath may be required.
(1)Proceed by separating the thermoelements, conductors,
8. Procedure
orthermoelementpairssothattheyarenotinelectricalcontact
8.1 Resistance Measurement:
with each other or with the sheath. The sheath should be
8.1.1 Ifepoxyresinhasbeenusedasasealant,makecertain
electrically connected to ground.
ithasfullycuredbeforeconductingthetest.Taketheresistance
(2)Connect the positive lead of the measuring instrument
measurementsinadrylocationatroomtemperature(22 65°C
to one of the thermoelements, conductors, or one of the
(72 6 10°F)
...