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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-02-01</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001303129</rptOwnerCik>
            <rptOwnerName>Locke Stanley P</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>V.P. Bus Dev St. Planning</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-02-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>787</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>51.6847</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18055</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12806.016</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>401(k)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Advance election, pursuant to the Company's Equity Compensation Plan, made at the time of the 2008 award to satisfy the tax-withholding obligation on vesting.</footnote>
        <footnote id="F2">Common Stock Units held in the Thomas &amp; Betts Company Stock Fund, an investment option in the Thomas &amp; Betts Corporation Employees' Investment Plan.  This information is based on a plan report as of February 2, 2011.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>W. David Smith, Jr.,  Attorney-in-Fact</signatureName>
        <signatureDate>2011-02-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2011-02-01
0000097854
THOMAS & BETTS CORP
TNB
0001303129
Locke Stanley P
8155 T&B BOULEVARD
MEMPHIS
TN
38125
0
1
0
0
V.P. Bus Dev St. Planning
Common Stock
2011-02-01
4
F
0
787
51.6847
D
18055
D
Common Stock
12806.016
I
401(k)
Advance election, pursuant to the Company's Equity Compensation Plan, made at the time of the 2008 award to satisfy the tax-withholding obligation on vesting.
Common Stock Units held in the Thomas & Betts Company Stock Fund, an investment option in the Thomas & Betts Corporation Employees' Investment Plan.  This information is based on a plan report as of February 2, 2011.
W. David Smith, Jr.,  Attorney-in-Fact
2011-02-03