Engineering corrective-action notice · PV-FUSE-01 · 9 November 2026

Murata DC voltmeter input-wire protection

Heating Rate Controller · PV1 / PV2 · DMR20-10-DCM-R-C · Existing reference: electrical issue R1.2

Confirmed current condition — corrective action OPEN. Neither PV1 nor PV2 has an external fuse installed on its voltmeter supply wiring. Do not authorise live heater operation until the missing input protection is resolved and the wider electrical checks are completed. This notice describes a requirement and approval process, not a fitted circuit or an approved wiring modification.

Manufacturer requirement

Murata's DMR20-10-DCM technical notes, section 4, specify an external, user-supplied 0.25 A / 300 V time-delay (time-lag) fuse for the supply wiring connected to the meter's TB1 (+) and TB1 (−) terminals, in accordance with applicable requirements. The meter's internal self-resetting fuse does not replace this external supply-wire protection.

The two-wire meters measure their own channel's external DC input. Their operating range is approximately 6–75 V DC; the heater system design envelope is 0–60 V DC. Murata specifies 18–22 AWG copper input conductors with suitable insulation; check its datasheet for terminal preparation, torque and installation requirements. These meters do not measure heater current.

Murata DMR20 installation datasheet — section 4 (page 2) →

Protection topology is not yet selected. The manufacturer's text addresses both the + and − input supply wires. The exact number and placement of fuseholders must be determined against the actual external DC source topology, earthing/return arrangement, accessible conductors and applicable requirements. Do not assume that one positive-leg fuse, a general-purpose AC fuseholder, or the 16 A heater breaker automatically satisfies the instruction. Check the complete fuse-and-holder DC voltage rating and prospective-fault-current interrupt capability.

Equipment and open actions

MeterExisting functionExternal protectionDisposition
PV1 / CH1External channel 1 DC supply voltage, TB1 (+/−)NOT FITTEDEngineering selection, installation and test required
PV2 / CH2External channel 2 DC supply voltage, TB1 (+/−)NOT FITTEDEngineering selection, installation and test required

Required design and inspection checks

  1. Document the actual input-wire routes and DC source reference/earthing arrangements separately for PV1 and PV2; photograph and identify meter TB1 and upstream connection points.
  2. Select and document the 0.25 A / 300 V time-delay fuse(s), fuseholder(s), required conductor protection, DC-rated voltage/interrupt capability, compatible wiring and physical mounting.
  3. With both mains and external DC sources isolated and proved dead, implement the approved modification and apply durable wire IDs/protection labels.
  4. Verify conductor continuity, correct meter polarity, branch protection, clearances, strain relief, connection security and absence of shorts. Check DC source voltage and current-limiting behaviour.
  5. Perform the approved functional and fault/protection checks; record results, reviewer and authorisation before permitting heater operation.
  6. Issue a new controlled drawing revision and update the 70-route schedule and terminal records after verifying the actual modified wiring. Do not insert hypothetical rows into R1.2.

Corrective-action acceptance record

Blank fields are intentional. This page is not an electronic sign-off system; printing it does not constitute acceptance.

Physical wiring and input protection selected______________________________________
Fuse / holder model(s), rating, DC interrupt rating and quantities______________________________________
PV1 fitted, inspected and tested______________________________________
PV2 fitted, inspected and tested______________________________________
Drawing / schedule issue and inspection report______________________________________
Competent reviewer, date and site approval______________________________________

Status: OPEN until all required evidence is recorded and accepted.