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Using Continuous Monitoring to Meet NERC PRC-005-6 for VRLA & VLA Station Batteries

Using Continuous Monitoring to Meet NERC PRC-005-6

November 03, 2025
7 min read

Using Continuous Monitoring to Meet NERC PRC-005-6 for VRLA & VLA Station Batteries

Scope: This is a straight, standards-first walkthrough of how to use continuous battery monitoring to meet NERC PRC-005-6 for station DC supplies with VRLA and VLA batteries. I map the required monitoring attributes in Table 1-4(f) to CellSPY telemetry/alarms and call out what monitoring does (and does not) eliminate in your maintenance program.

What PRC-005-6 Actually Cares About

  1. Protection System Maintenance Program (PSMP):
    You must have a PSMP that specifies whether each Component Type is maintained time-based or performance-based, and—if you use monitoring to adjust intervals—your PSMP must list the applicable monitoring attributes consistent with the standard’s tables.

  2. Station DC supply monitoring attributes:
    For station DC supplies, NERC defines a set of monitoring attributes that, if present with alarming, remove the need for certain periodic verifications. That’s codified in Table 1-4(f): Exclusions for Protection System Station DC Supply Monitoring Devices and Systems.

The rest of this post maps Table 1-4(f) to how I implement it with monitoring—then summarizes evidence you hand an auditor.

Table 1-4(f): Attribute-by-Attribute Mapping (What Monitoring Eliminates)

Everything below is straight from Table 1-4(f). I’m quoting the “If you have X monitoring/alarming, then Y periodic task is not required” logic, then showing the monitoring implementation.

1) Battery charger high/low voltage monitoring & alarming

NERC:
If the station DC supply has high and low voltage monitoring/alarming on the charger voltage (detecting over-voltage and charger failure) → “No periodic verification of station dc supply voltage is required.”

Monitoring implementation:

  • Log charger output voltage continuously; alarm on high/low.
  • Include an exception log for events.
  • CellSPY exposes millivolt-resolution DC measurement to support tight bands.

2) Electrolyte level monitoring & alarming for every cell (VLA)

NERC:
If a battery-based station DC supply has electrolyte level monitoring/alarming in every cell → “No periodic inspection of the electrolyte level for each cell is required.”

Monitoring implementation:

  • Per-cell liquid-level sensor (mm) with alarm.
  • CellSPY measures actual electrolyte level in mm; include an alarm and a trend to document compliance.

3) Unintentional DC ground monitoring & alarming

NERC:
If the station DC supply monitors/alarms for unintentional DC grounds → “No periodic inspection of unintentional dc grounds is required.”

Monitoring implementation:

  • Ground-detector channel tied into the monitoring platform; alarm history retained with timestamps.
  • Report the measurement method and threshold in the PSMP mapping (auditors look for the attribute + alarm, not a brand).

4) Float voltage monitoring & alarming

NERC:
If the station DC supply monitors/alarms charger float voltage to ensure correct float is applied → “No periodic verification of float voltage of battery charger is required.”

Monitoring implementation:

  • Configure a float window per OEM, log deviations, and show alarm acknowledgements in the audit pack.
  • Use the same mV-resolution measurement capability noted above.

5) Battery string continuity monitoring & alarming

NERC:
If the battery-based station DC supply monitors/alarms string continuity → “No periodic verification of the battery continuity is required.”

Monitoring implementation:

  • String-level continuity check with alarm; document method (open-string detection, fuse/strap continuity diagnostic) and retain event logs.

6) Intercell / terminal connection detail resistance monitoring & alarming (entire battery)

NERC (Table 1-4(f) continuation, summarized in vendor mapping):
If the battery system monitors/alarms intercell and/or terminal connection detail resistance → “No periodic verification of those resistances is required.”

Monitoring implementation:

  • Establish installation baselines for strap/terminal resistance.
  • Trend deltas; alarm on threshold excursions (e.g., ≥ +20% vs. baseline aligns with IEEE corrective-action practice).
  • Store the pair-mapping for posts so the geometry is consistent quarter-to-quarter.

7) Internal ohmic value / float current monitoring & alarming with baseline evaluation (VRLA & VLA)

NERC (Table 1-4(f) continuation, summarized in vendor mapping):
If a VRLA or VLA station battery has internal ohmic value or float current monitoring with alarming and evaluation relative to baseline (for every cell/unit) → “No periodic ‘baseline evaluation’ of cell/unit measurements is required” to verify the station battery can perform as manufactured.

Monitoring implementation:

  • Capture per-cell ohmic values (impedance/resistance/conductance) on a fixed cadence.
  • Maintain a site baseline; compute deviation per cell and alarm when significant.
  • CellSPY’s design uses a ~1 A, fraction-of-a-second stimulus for ohmic reads (not “tens of amps”), limiting test-induced stress—this matters for always-on monitoring.

8) Internal ohmic value monitoring & alarming (each cell/unit)

NERC (Table 1-4(f) continuation, summarized in vendor mapping):
If a VRLA or VLA station battery monitors/alarms each cell/unit internal ohmic value →
“No periodic inspection by measuring all cells’ ohmic values is required.”

Monitoring implementation:

  • Same as above, but emphasize per-cell coverage and alarming.
  • Include instrument specs (range/resolution/accuracy) in the PSMP appendix; CellSPY specs are listed for voltage, temperature, impedance, and level.

What Monitoring Does Not Remove

  • Capacity / performance testing obligations in PRC-005-6 still exist and are table-driven by Component Type (i.e., your PSMP must meet the minimum activities and maximum intervals in the applicable tables).
  • Monitoring removes specific periodic verifications where Table 1-4(f) grants exclusions; it does not erase capacity-test requirements elsewhere in the standard.
  • Keep your PSMP wording precise and tie each exclusion to Table 1-4(f).

PSMP Language: How I Document This

Structure:

  • Identify the Component Type (“Protection System Station DC Supply using VRLA/VLA”) and state time-based for capacity tests; monitoring-based exclusions for attributes listed in Table 1-4(f).
  • For each attribute above, reference the alarm name, threshold, and evidence (report names, log retention). This satisfies the “applicable monitoring attributes” requirement when you use monitoring to alter periodic work.

Evidence pack (what I hand the auditor):

  • One-click NERC report (scheduled) summarizing: charger high/low events, float violations, continuity events, ground-detector events, electrolyte levels (VLA), per-cell ohmic trends, and strap/terminal resistance deltas with baseline references. The vendor sheet explicitly calls out one-click NERC-compliant reporting.
  • Operating specs (range/resolution/accuracy) for voltage, temperature, impedance, and level—inserted in the PSMP appendix for traceability.
  • Alarm history and acknowledgements with timestamps for any attribute excursions (proves the “alarming” part of the exclusion).
  • Baseline records for ohmic and connection-resistance values.

Design Notes (Why Always-On Monitoring Doesn’t Create a New Problem)

  • On float, monitors are effectively powered by the charger, not draining the host jar; the charger sources the extra milliamps and the string voltage stays at setpoint.
  • In storage, enable sleep/low-power: added discharge is about 0.2%/month on 150 Ah—negligible vs. normal 3–3.5%/month VRLA self-discharge—and you still get a 5-minute voltage/temperature heartbeat to prevent deep discharge.
  • Ohmic reads use a ~1 A momentary stimulus, not tens of amps, avoiding monitor-induced aging. Field history supports that the system has not caused premature failures; failures we do see typically trace to ripple or thermal issues—which monitoring surfaces.

Bottom Line (VRLA & VLA)

  • If you instrument the attributes in Table 1-4(f) with continuous monitoring + alarming, NERC explicitly waives the listed periodic verifications (charger voltage, float voltage, electrolyte level (VLA), unintentional DC grounds, battery string continuity, intercell/terminal resistance, and per-cell ohmic values with baseline evaluation). That’s the standard—not interpretation.
  • Your PSMP must say exactly how you’re using monitoring (attributes + alarms) to meet those exclusions and still cover any remaining time-based work such as capacity testing on the intervals applicable to your Component Type.
  • With a proper implementation, you shrink clipboard rounds down to exceptions and tests that actually matter, and you walk into audits with automated reports and raw data instead of hand-entered logs.