[RE-wrenches] Clamping CTs Around Live Conductors - White Paper References and/or Anecdotal Information Requested

[Blake Gleason]

Hi Sam,

The "secondary" of the CT is the side connected to the meter.  The primary
is the side that gets wrapped around the conductor or busbar on which you
are measuring current.  So yes, make sure that the secondary is connected
to its "burden" (the meter, or a shorting block) before inducing any
current in the primary.  With a split-core CT, or a handheld clamp meter
(as you noted), you can install the CT around a conductor while the
conductor is in service.  Just make sure the secondary is already
connected.  (I'm sure there is some exception for certain types of
sensitive meters, but I've never had a problem.  And yes, as you say, this
is assuming that you are using proper PPE, etc for working around live
circuits.)

Blake


On Fri, Mar 8, 2024 at 8:31 AM Sam Haraldson via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> Dear Wrenches,
>
> I'm trying to determine whether or not it is acceptable to clamp
> monitoring CTs around hot conductors.  For this question please assume the
> CT wires are landed in their proper terminals and that proper PPE/safety
> protocol is employed.  The monitored currents in question are residential
> services of 600A or less. Is there harm to the CT by clamping them around a
> hot conductor?
>
> The physics of a CT from my brief research states that,* "...the
> secondary of a current transformer should not be disconnected from its
> burden while current is in the primary, as the secondary will attempt to
> continue driving current into an effective infinite impedance potentially
> generating high voltages and thus compromising operator safety."  *This begs
> the question how I can use my DMM clamp meter w/o causing damage or is the
> italicized text related to very high current applications from the world of
> medium and high voltage equipment and not as pertinent to the residential
> and light commercial world I work in?
>
> I suspect this is a no-brainer answer for the seasoned professionals on
> the list and I'll be most appreciative for any proof (no matter how brief)
> you wish to share.  Ultimately we wish to minimize the need to perform
> power shut downs and interrupt client service when installing load
> monitoring CTs to a home but only wish to do so if it is part of a safe and
> non equipment-harming process.
>
> Sincerely,
> Sam Haraldson
>
> [image: OnSite Energy]
> <https://www.google.com/url?q=https://onsiteenergyinc.com/&source=gmail-html&ust=1677085494759000&usg=AOvVaw2NN36GoM1sjnS4RdC8Svt->
>
> SAM HARALDSON
> Field Operations Director
> (406) 551-6135
> 1515 N. Rouse Ave Bozeman, MT 59715
> Locally owned and operated since 2012
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-- 

Blake Gleason, PE | Employee Owner
Director of Innovation and Technical Excellence
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