21-day window ending 2026-05-06. Verdigris MCP & Signals Engine. IEEE Std 519-1992 anchored thresholds.
A 20-day rectifier harmonic survey across the Apex Telecom portfolio, run on the Verdigris MCP and Signals Engine. Findings are gated against IEEE Std 519-1992 individual current harmonic limits, scoped per panel, and bucketed into two operational categories.
Category B — whole-DC-plant patterns (60 breakers, 5 sites). Most or all rectifiers on a single DC plant elevated together. The fault path is plant-specific (model, vendor, vintage, or upstream feeder); the building’s incoming voltage supply is clean. Engineering investigation, not module replacement.
Category A — unit-level investigations (11 breakers, 4 sites). One rectifier or a single redundant pair sits well above same-site peers running at comparable load, and harmonic content exceeds the IEEE 519 individual limit with margin. Validation candidate.
This report does not recommend equipment replacement. Each Category A finding has a physical validation step; each Category B finding identifies which DC plants exceed IEEE 519 individual-harmonic limits and where the fault path is.
Threshold anchor — IEEE Std 519-1992. IEEE Std 519-1992 § 10.3, Table 10.3 (Current Distortion Limits for General Distribution Systems, 120 V to 69 kV) caps individual current harmonic distortion at the point of common coupling based on the system’s short-circuit-to-load-current ratio. For typical telecom Central Distribution Center (CDC) supply (Isc/IL between 100 and 1,000), the standard limits the 5th and 7th harmonic individually to 12% of maximum demand current and total demand distortion (TDD) to 15%.
| Reference | Healthy peer | IEEE 519 indiv h<11 | IEEE 519 TDD | Gate A (this report) | Worst flagged |
|---|---|---|---|---|---|
| Threshold | 5 to 7% THD-57 | 12% | 15% | 18% THD-57 | 37% THD-57 |
The Gate A value of 18% THD-57 sits just above the level where both the 5th and 7th orders simultaneously exceed the IEEE 519 individual harmonic limit (sqrt(0.12² + 0.12²) ≈ 17%). Worst-case flagged units (Site 8 DC2-1) read individual 5th-harmonic content of 34% of fundamental, roughly 3x the IEEE 519 individual limit.
Detection gates. Only samples where the rectifier was active with nominal load are counted (applied before any harmonic aggregation):
Detection — flag if either gate trips on the surviving samples:
Before treating the harmonic content as rectifier-side, we measured supply voltage distortion across each affected site, then compared rectifier-side current distortion to mechanical loads (CRAC, HVAC, AHU, chillers, pumps) on the same supply. IEEE Std 519-1992 § 11, Table 11.1 caps voltage THD at the point of common coupling at 5.0% below 69 kV. The mechanical comparison only counts samples while the equipment is active with nominal load (loaded ≥ 70% of the panel’s near-peak level), the same filter applied to the rectifier analysis.
| Site | Pool | V THD | I THD-57 | n | Avg load (A) | Rect / Mech |
|---|---|---|---|---|---|---|
| Site 1 | Mechanical | 2.1% | 3.8% | 3 | 30.1 | |
| Rectifier | 2.2% | 34.6% | 4 | 35.4 | 9.1x | |
| Site 2 | Mechanical | 3.0% | 4.8% | 5 | 50.0 | |
| Rectifier | 4.0% | 23.9% | 16 | 26.2 | 5.0x | |
| Site 3 | Mechanical | 3.2% | 15.2% | 2 | 56.9 | |
| Rectifier | 3.1% | 28.2% | 21 | 9.8 | 1.9x | |
| Site 4 | Mechanical | 4.6% | 15.3% | 2 | 11.3 | |
| Rectifier | 3.8% | 18.5% | 23 | 12.1 | 1.2x | |
| Site 5 | Mechanical | 1.5% | 8.5% | 14 | 26.8 | |
| Rectifier | 1.5% | 9.4% | 30 | 12.0 | 1.1x | |
| Site 6 | Mechanical | 1.5% | 11.1% | 8 | 25.7 | |
| Rectifier | 2.3% | 13.1% | 8 | 20.4 | 1.2x | |
| Site 7 | Mechanical | 1.0% | 8.2% | 7 | 25.7 | |
| Rectifier (whole-site avg) | 1.2% | 6.5% | 25 | 6.5 | 0.8x | |
| Site 8 | Mechanical | 3.1% | 15.6% | 3 | 189.6 | |
| Rectifier (whole-site avg) | 2.9% | 7.5% | 84 | 16.8 | 0.5x | |
| Site 9 | Mechanical — no breakers tagged | — | — | — | — | |
| Rectifier | 2.3% | 11.2% | 27 | 14.5 | — |
Voltage THD comes in between 1.0% and 4.6% at every site, under the IEEE 519 5% limit at the point of common coupling. The supply is clean. The harmonic content is being generated downstream of the building service entrance.
At Sites 1, 2, and 3 the loaded mechanical pool draws 4 to 15% current THD-57 while the loaded rectifier pool draws 24 to 35% on the same supply. The 1.9 to 9.1x contrast is direct evidence the rectifiers are the harmonic source at those sites, not the supply, and not the building’s other non-linear loads.
At Sites 5, 6, 7, and 8 the rectifier whole-site average is dragged down by a large clean bank, so the rectifier-vs-mechanical site average looks similar or inverted. The unit-level findings at those sites are still real, they show up in the per-DC-plant comparison below, where the affected DC plant reads 3 to 6x higher than other DC plants on the same site.
Five sites with most or all rectifiers on a single DC plant elevated together. Three of them (Sites 2, 4, 9) host other DC plants on the same site that read normally, useful signal that the issue is plant-specific, not site-wide. Sites 1 and 3 have only the affected DC plant on site, so within-site control comparison isn’t available.
| Site | DC plant | Breakers | Avg THD-57 | Min | Max | Avg active A |
|---|---|---|---|---|---|---|
| Site 3 | Plant A-1 | 21 | 0.282 | 0.279 | 0.292 | 9.8 |
| (no other DC plant on site) | — | — | ||||
| Site 2 | Plant A-2 | 7 | 0.269 | 0.267 | 0.270 | 23.6 |
| Plant A-1 | 7 | 0.268 | 0.267 | 0.269 | 23.6 | |
| Plant B-2 (different) | 1 | 0.033 | 0.033 | 0.033 | 44.1 | |
| Plant B-1 (different) | 1 | 0.030 | 0.030 | 0.030 | 44.2 | |
| Site 4 | Plant A-1 | 19 | 0.211 | 0.032 | 0.268 | 6.8 |
| Plants B-1 / C-1 / D-1 | 4 | 0.051 to 0.078 | 0.051 | 0.078 | 32.7 to 41.1 | |
| Site 9 | Plant A-1 | 23 | 0.130 | 0.035 | 0.252 | 6.4 |
| Plants B / C / D / E | 4 | 0.052 to 0.056 | 0.052 | 0.056 | 36.0 to 42.0 | |
| Site 1 | Plant A SECONDARY | 1 | 0.403 | 0.403 | 0.403 | 26.8 |
| Plant B PRIMARY | 1 | 0.356 | 0.356 | 0.356 | 35.2 | |
| Plant B SECONDARY | 1 | 0.307 | 0.307 | 0.307 | 39.9 | |
| (no other DC plant on site) | — | — |
Sites 2, 4, and 9 show the affected DC plant approximately 5x higher THD-57 than other DC plants on the same site running comparable load. Combined with clean voltage at the building, the fault path is plant-specific: model, vendor, vintage, or the feeder between the building main and the affected rectifier panel. Sites 1 and 3 lack a within-site control plant, so within-site causal narrowing is not possible from the data alone.
Eleven breakers across four sites where the rectifier(s) sit well above same-site peers operating at comparable load and exceed the IEEE 519 individual harmonic limit. Each is a candidate for low-cost physical validation.
| Site | Unit / cluster | Brks | Avg THD-57 | Avg A | Peer THD-57 | Ratio |
|---|---|---|---|---|---|---|
| Site 8 | DC2-1 redundant pair | 4 | 0.355 | 18.9 | 0.062 (50+ peers) | 5.7x |
| Site 6 | Plant B-2 whole plant | 4 | 0.189 | 20.8 | 0.073 (sister plant, 4 units) | 2.6x |
| Site 5 | Plant B-side (3 modules) | 1 (+5 sub-gate) | 0.180 | 21.7 | 0.033 (Plant 3-03 / 3-04) | 5.5x |
| Site 7 | Rectifier 7 on Plant 02 | 1 | 0.369 | 6.3 | 0.029 (Plant TYCO-01 + rest) | 12.7x |
Sites 5 and 6 are DC-plant common cause: multiple rectifiers on the same plant carry the elevated reading and share the same waveform shape. The cause is shared infrastructure on the affected DC plant: AC-side input filter or isolation transformer, the conductor between the building main and the panel, the common DC output bus and its filter capacitor bank, or a shared load-side element reflecting ripple back through every module. Replacing individual rectifier modules will not resolve them. Site 8 DC2-1 and Site 7 Rectifier 7 sit on rectifiers whose same-site siblings are clean, so the fault is on the module side at those two.
Two physical rectifier modules (DC2-1A and DC2-1B), each metered on its two redundant AC inputs. All four metering points read 33 to 37% THD-57 over 21 days while loaded to approximately 19 A, against a same-site peer median of 6% across 50+ other rectifier breakers running at comparable or higher load. The 5th harmonic alone is approximately 34% of fundamental, roughly 3x the IEEE 519 individual harmonic limit.
Figure 1. Daily THD-57 (avg + max) on Rectifier DC2-1A while loaded, 21 days. The +0.2 percentage-points-per-day trend trips Gate B, on top of the chronic exceedance trip from Gate A. Site peer median (~6%) and IEEE 519 individual harmonic limit (12%) shown for reference.
Figure 2. Pro Capture current. DC2-1A (subject) vs DC2-3A (clean same-site peer), both loaded ~22 A, aligned to the 60 Hz fundamental for shape comparison. The peer draws a recognizable double-pulse 6-pulse signature; the subject draws chopped, asymmetric current with deep notches.
Three rectifier modules on the affected DC plant share the same chopped current waveform morphology. Only one of three clears the Gate-A 18% threshold on the 20-day aggregate; the second sits at 16% (just below); the third at 12%. The Pro Capture from 2026-05-05 (figure below) shows all three are essentially the same rectifier signature at different load levels. The gate sliced through a continuous distribution rather than isolating one bad unit. The site’s clean reference rectifier (4% THD-57 across all three phases) is included at the bottom of the figure for the same instant.
Figure 3. Per-rectifier Pro Capture on the affected DC plant (3 modules x 2 AC feeds x 3 phases = 18 channels) plus the clean reference rectifier (all three phases) captured at the same instant 2026-05-05T13:16:09Z. Waveforms aligned to the 60 Hz fundamental zero-crossing for shape comparison.
What this means. Three rectifier modules cannot independently degrade in lockstep with identical waveform morphology. The cause is a shared element on the DC plant, something every module on the affected plant sees and the clean reference plant does not. Candidates include the AC-side input filter or isolation transformer, the conductor between the building main and the panel, the common DC output bus and its filter capacitor bank, or a shared load-side element on the DC distribution that reflects ripple back through every module. The site’s voltage supply is clean (1.5% THD), so it is not arriving from upstream of the building service. Replacing individual rectifier modules on this plant will not change the reading.