LTE DIN Rail Analog Transmitters with Ethernet Communication and Analog outputs for True RMS AC Voltage and Current Applications

LTE True RMS AC Transmitter Frequently Asked Questions

Why does the accuracy calculation footnote treat the 600V range as "2000V (20,000 counts)" instead of using the range's actual 600V ceiling?

Documented footnote specifically applies this substitution for accuracy calculation purposes only, without changing the transmitter's actual measurable range or displayed reading — this is consistent with the underlying signal conditioner circuitry for that range being built around a higher internal full-scale reference than the certified/displayed 600V ceiling, so the accuracy formula is documented as referencing that internal full-scale figure rather than the smaller displayed range ceiling.

Why does crest factor increase from 3.0 at full scale to as high as 300 at 1% of full scale, rather than staying constant?

Documented specification lists crest factor (Vp/Vrms) as varying specifically with signal amplitude — since crest factor describes how much a signal's peak can exceed its RMS value without the transmitter losing accuracy, and the transmitter's peak-handling headroom is documented as essentially fixed in absolute terms, a much smaller RMS signal (1% of full scale) can tolerate a proportionally much larger peak-to-RMS ratio before hitting that same fixed peak-handling ceiling, which is documented as the reason crest factor rises sharply at low signal amplitudes.

Why does selecting the 10 MΩ input resistance factory special specifically reduce maximum frequency from 5 kHz to 1 kHz, and only for three of the six voltage ranges?

The page documents this tradeoff (higher input impedance in exchange for reduced maximum frequency on three ranges) as a factory special option without detailing the internal circuit reason — this is consistent with higher input resistance typically requiring circuit component changes that affect high-frequency response, a common tradeoff in precision signal conditioning, though the page itself presents this as a documented specification rather than explaining the underlying circuit design choice.

Does AC coupling change the transmitter's documented true RMS accuracy figures compared to DC coupling?

The page documents the same accuracy figures (0.03% FS, with the same frequency and amplitude qualifiers) as applying across the transmitter's DC-to-5kHz measurement range without listing separate accuracy figures specifically for AC-coupled versus DC-coupled operation — AC and DC coupling are documented as a jumper-selectable choice about which signal content passes through, applicable to measuring different specific signals like DC ripple versus DC value, without altering the transmitter's underlying documented RMS accuracy.

Why does the capture behavior change from multi-cycle averaging above 50/60 Hz to single-cycle capture between 3 Hz and 50/60 Hz, and 333 ms capture below 3 Hz?

Documented description specifically ties capture strategy to signal frequency relative to the power line frequency — above 50/60 Hz, multiple complete cycles fit within a reasonably short time window and can be documented as averaged together; between 3 Hz and 50/60 Hz, a single complete cycle already takes a meaningful amount of time, so one cycle is documented as captured; below 3 Hz, a full cycle would take too long, so the documented 333 ms interval capture provides a practical, bounded update rate instead of waiting for a complete slow cycle.

Does the transmitter's documented high common mode rejection eliminate the need for an isolated CT when placing a shunt on the high side of the line?

Not necessarily — documented description specifically states high CMR "allows for stable readings" with high-side shunts, addressing the transmitter's own measurement stability in that configuration, but doesn't document this as eliminating separate electrical safety or isolation requirements for high-side shunt installations; high CMR is a documented measurement-stability specification, distinct from whatever isolation practices a given high-side installation may independently require for safety.

Does this LTE True RMS transmitter's documented Modbus TCP-only protocol limit compatibility compared to the RS232/RS485 LT Series True RMS variant?

Yes — this page documents Modbus TCP specifically as the supported Ethernet Data I/O protocol at digital address 247, while the LT Series serial variant is documented elsewhere as separately supporting Modbus RTU/ASCII and Laurel Custom ASCII; a control system needing a protocol other than Modbus TCP would need to reference the LT Series serial variant rather than this LTE Ethernet variant.

Does the 5A built-in shunt's documented 0.01 ohm value need to match a specific current transformer's rated burden?

The page documents the 0.01 ohm shunt as fixed, built-in hardware sized to accept 5A CT output directly, without stating a specific compatible burden rating range for external CTs — since the shunt value is fixed rather than adjustable, whether a given 5A-output CT is well-matched to this specific 0.01 ohm burden is a factor to verify against that CT's own manufacturer specifications rather than something the page documents as universally compatible.

Does selecting the Extended main board for custom curve linearization change this transmitter's documented true RMS accuracy specifications?

No — documented Extended board capability (custom curve linearization, rate from consecutive readings) is described as an additive processing feature layered on top of the underlying measurement; the documented true RMS accuracy figures (0.03% FS, crest factor specifications) apply to the underlying signal conditioning regardless of whether the Standard or Extended main board is selected.

Does the documented 0-16.7 ms response time represent the time for one full reading, or the time before any output is available at all?

Documented phrasing specifically describes this as the time "after completion of one input signal cycle" for a true RMS reading to become available — this is consistent with the 0-16.7 ms window representing processing time following a complete cycle capture (16.7 ms corresponding to one full cycle at 60 Hz), rather than representing dead time before the transmitter begins capturing any signal at all.

Current Transformer Burden Resistor & Safety Questions From the Field

Why is it specifically dangerous to open-circuit a current transformer's secondary while the primary conductor is still energized?

Documented explanation specifically describes a CT secondary attempting to continue driving current into what becomes an effectively infinite impedance once open-circuited — with primary current still flowing, documented analysis shows core flux increasing dramatically due to the loss of the secondary's normal opposing magnetic effect, causing secondary voltage to rise to a level that can exceed the insulation's breakdown rating, with one documented example specifically citing open-circuit secondary voltage potentially exceeding 1500V RMS on a 1000:5 CT.

What specific function does a burden resistor serve in a current-output CT's secondary circuit?

Documented explanation specifically describes the burden resistor as converting the CT's secondary current into a proportional voltage signal, while also keeping the secondary circuit closed to prevent the dangerous open-circuit voltage rise described above; the burden value is documented as chosen specifically to stay low enough to avoid CT core saturation while still producing a usable voltage signal.

Is there a documented practical formula for calculating a burden resistor's maximum allowable value given a CT's rated VA and lead wire resistance?

Yes — one documented worked example specifically calculates maximum instrument impedance for a 5A CT with a 5VA burden rating and #14 AWG leads (with a documented round-trip lead resistance of approximately 1.27Ω) as 5VA/25A minus 1.27Ω, equal to about 0.73Ω; this documented calculation illustrates how lead wire resistance must specifically be subtracted from the available burden budget before sizing the burden resistor itself.

Do standards bodies specifically require a documented safe time window for voltage-limiting protection during an accidental CT open-circuit event?

Yes — documented guidance specifically cites IEEE C57.13 as advising that voltage-limiting devices fitted to CT secondary windings should be able to withstand an open-circuit condition for a full minute without damaging the secondary circuit, providing a documented, standards-referenced time benchmark for protective device performance during this specific fault condition.

Is there a documented safe practice for disconnecting instruments from an in-service CT secondary circuit?

Yes — documented guidance specifically recommends always short-circuiting a CT's secondary terminals before removing any connected instrument, using shorting blocks or test switches with integral shorting contacts specifically designed for this purpose; this documented practice keeps the secondary circuit closed at all times during the disconnection process, preventing the momentary open-circuit condition that would otherwise occur.

Does a "voltage output" style CT (with the burden resistor built into the CT body) eliminate the open-circuit hazard that applies to current-output CTs?

Yes, specifically for that hazard — documented explanation specifically notes that CTs with an internal burden resistor (the voltage-output type) are inherently protected against developing the dangerous open-circuit voltages that a bare current-output CT can produce if its external burden connection is lost, since the burden resistor's protective function is built permanently into the CT itself rather than depending on an external component staying connected.

Does routine maintenance genuinely risk an accidental CT open-circuit condition, or is this mainly a design-stage concern?

It's a genuine, documented in-service risk, not just a design-stage concern — documented guidance specifically lists loose connections, damaged wiring, and added devices in the secondary circuit as real-world causes of increased burden or accidental open-circuiting during a CT's operational life, and specifically recommends periodic inspection of the secondary circuit for exactly these issues as an ongoing maintenance practice.

Is CT polarity (dot convention) documented as mattering for basic single-CT current measurement, or only for more complex protection schemes?

Documented explanation specifically ties polarity most directly to multi-CT protection schemes — citing 3-phase protection relays and differential protection schemes as specific applications where incorrect CT polarity is documented as causing improper restraint or incorrect directional overcurrent operation; while polarity still matters generally for correct current direction convention, the documented consequences of a polarity error are specifically most significant in these multi-CT protective relay contexts.