<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0305</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-05-16</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0000097854</issuerCik>
        <issuerName>THOMAS &amp; BETTS CORP</issuerName>
        <issuerTradingSymbol>TNB</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001376828</rptOwnerCik>
            <rptOwnerName>Hajj Imad</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>8155 T&amp;B BOULEVARD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MEMPHIS</rptOwnerCity>
            <rptOwnerState>TN</rptOwnerState>
            <rptOwnerZipCode>38125</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Sr. V.P. Int'l &amp; Ops Dev.</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-05-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>9232</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>72.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock Units</value>
            </securityTitle>
            <transactionDate>
                <value>2012-05-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>65097.489</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>72.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>401(k)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Employee Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>34.57</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-05-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>8597</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>37.43</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-12-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>8597</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Employee Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>25.29</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-05-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>9895</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>46.71</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2020-12-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>9895</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Employee Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>19.30</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-05-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>13686</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>52.70</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <value>2021-12-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>13686</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Disposition of shares pursuant to the Agreement and Plan of Merger by and among ABB Ltd., Edison Acquisition Corporation and Thomas &amp; Betts Corporation for a cash payment of $72.00 per share.</footnote>
        <footnote id="F2">Disposition of common stock units in the Thomas &amp; Betts Stock Fund, an investment option in the Thomas &amp; Betts Employees' Investment Plan pursuant to the Agreement and Plan of Merger by and among ABB Ltd., Edison Acquisition Corporation and Thomas &amp; Betts Corporation for a cash payment of $72.00 per share.  The Plan Trustee provided a conversion factor of .329.  Accordingly, Mr. Hajj's disposition was equivalent to approximately 21,623.072 shares.</footnote>
        <footnote id="F3">This option, which provided for vesting in three equal annual installments beginning December 2, 2010, was converted in the merger between ABB Ltd., Edison Acquisition Corporation and Thomas &amp; Betts Corporation into the right to receive a cash payment of $72.00 minus the exercise price.</footnote>
        <footnote id="F4">This option, which provided for vesting in three equal annual installments beginning December 1, 2011, was converted in the merger between ABB Ltd., Edison Acquisition Corporation and Thomas &amp; Betts Corporation into the right to receive a cash payment of $72.00 minus the exercise price.</footnote>
        <footnote id="F5">This option, which provided for vesting in three equal annual installments beginning December 7, 2012, was converted in the merger between ABB Ltd., Edison Acquisition Corporation and Thomas &amp; Betts Corporation into the right to receive a cash payment of $72.00 minus the exercise price.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>W. David Smith, Jr.,  Attorney-in-Fact</signatureName>
        <signatureDate>2012-05-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "edgar.xml", "description": "PRIMARY DOCUMENT", "secsgml_size_bytes": 12786}]
X0305
4
2012-05-16
1
0000097854
THOMAS & BETTS CORP
TNB
0001376828
Hajj Imad
8155 T&B BOULEVARD
MEMPHIS
TN
38125
0
1
0
0
Sr. V.P. Int'l & Ops Dev.
Common Stock
2012-05-16
4
D
0
9232
72.00
D
0
D
Common Stock Units
2012-05-16
4
D
0
65097.489
72.00
D
0
I
401(k)
Employee Stock Option (Right to Buy)
34.57
2012-05-16
4
D
0
8597
37.43
D
2019-12-02
Common Stock
8597
0
D
Employee Stock Option (Right to Buy)
25.29
2012-05-16
4
D
0
9895
46.71
D
2020-12-01
Common Stock
9895
0
D
Employee Stock Option (Right to Buy)
19.30
2012-05-16
4
D
0
13686
52.70
D
2021-12-07
Common Stock
13686
0
D
Disposition of shares pursuant to the Agreement and Plan of Merger by and among ABB Ltd., Edison Acquisition Corporation and Thomas & Betts Corporation for a cash payment of $72.00 per share.
Disposition of common stock units in the Thomas & Betts Stock Fund, an investment option in the Thomas & Betts Employees' Investment Plan pursuant to the Agreement and Plan of Merger by and among ABB Ltd., Edison Acquisition Corporation and Thomas & Betts Corporation for a cash payment of $72.00 per share.  The Plan Trustee provided a conversion factor of .329.  Accordingly, Mr. Hajj's disposition was equivalent to approximately 21,623.072 shares.
This option, which provided for vesting in three equal annual installments beginning December 2, 2010, was converted in the merger between ABB Ltd., Edison Acquisition Corporation and Thomas & Betts Corporation into the right to receive a cash payment of $72.00 minus the exercise price.
This option, which provided for vesting in three equal annual installments beginning December 1, 2011, was converted in the merger between ABB Ltd., Edison Acquisition Corporation and Thomas & Betts Corporation into the right to receive a cash payment of $72.00 minus the exercise price.
This option, which provided for vesting in three equal annual installments beginning December 7, 2012, was converted in the merger between ABB Ltd., Edison Acquisition Corporation and Thomas & Betts Corporation into the right to receive a cash payment of $72.00 minus the exercise price.
W. David Smith, Jr.,  Attorney-in-Fact
2012-05-17