Wiring requirements of instrument current transformers

Question

Hi All 
 I am looking for clarification on a wiring arrangement I have always considered good practice and commonly used, versus an arrangement I was recently informed of. 
 To my knowledge, if the secondary connections of an instrument current transformer are not connected and there is a primary current flowing on the measured circuit, then the secondary circuit of the current transformer should be shorted-out (i.e. a 'jumper' connection installed across S1 and S2 terminals of the current transformer), the S2 terminal usually also being connected to protective earth. 
 Whilst I don't have any reference to specific current transformer OEM wiring diagrams to refer, the general recommendation is to always ensure this arrangement due the hazards associated with a high-voltage that is generated across the secondary terminals (as the secondary current attempts to drive through a high impedance) when the circuit being measured has current flowing, as this will generally lead to failure (malfunction, breakdown of insulation, and potentially a fire) of the current transformer, and a hazardous voltage present at the secondary terminals up until that point in time. 
 Recently I was informed that an alternative way of dealing with this is to do away with the jumper wire, and simply earth the S2 terminal. 
 I am not convinced this is a safer or more effective solution and wondered whether anyone has an opinion on the suitability of the former versus the latter arrangement 
 Thanks in advance 
 Neil W

Jam · Accepted Answer

It depends on the CT. 
 With a &ldquo;conventional&rdquo; CT with a current output (eg 100/5A or whatever) then I agree that open circuit secondaries should be avoided for fear of damage or danger. 
 However other constructions may differ. For example voltage output CTs are normal CTs with a defined burden in shunt built-in. As well as turning the output into a voltage, this means you can&rsquo;t open-circuit the actual CT as the internal burden is always present.

AJJewsbury · Answer

I'd agree with the above - similarly some consumer grade CTs have built in Zener diodes (presumably a pair back-to-back) that'll conduct and effectively do the shorting out for you automatically if the output voltage becomes excessive. 
 - Andy.

mapj1 · Answer

'just earthing' is not going to help, unless the other end of the winding is already earthed. The problem is that without a load ("burden") then the available voltage on the secondary is essentially only limited by the core saturation and for very good mechanical reasons, to get the one pass of the big "wire" up the middle, the core is oversized by normal transformer standards. Because of this, while in correct operation the inserted impedance is a fraction of an ohm, being the transformed burden value, without that load, the transformer suddenly becomes an inductor capable of dropping a goodly fraction of a volt per turn at 50z and more for the higher frequency components associated with load switching, so truly lethal voltages appear on the output, with hundreds or even a few thousand turns and with enough energy available to give it serious attitude, at least for long enough to injure a person or damage the secondary insulation. 
 Worse, if you then try and disconnect it with current flowing, you the stored I2L energy back, which again is painful.... 
 In summary unless there is some sort of solidly connected load, the only guaranteed safe current through all but the smallest current transformer is zero. 
 And personally I really like the design with the idiot traps in the form of the anti-series pair of Zeners - it's saved my electronics more than once. 
 Mike.

Wiring requirements of instrument current transformers

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