Insulation Fault on a 5 kWp Rooftop That Appears Only After Rain
A 4.98 kWp rooftop system on a school building has twelve 415 Wp modules in two strings of six. Each string has Voc 227 V and the array has Voc 454 V. It feeds a 5 kW single-phase transformerless grid-tied inverter mounted in the staircase room; the manual states the inverter will not close its AC relay unless insulation resistance is at least 1 megaohm. During the monsoon the inverter logs an insulation fault before dawn and after rain, then runs normally by about 10:00 on dry days.
With the inverter DC switch off, both string fuses pulled and the DC isolator open, a 1000 V insulation tester gives: string 1 positive-to-earth 0.4 megaohm, string 1 negative-to-earth 18 megaohm, string 2 both poles above 50 megaohm. Earth electrode resistance is 1.8 ohm. The module frames are clamped to GI rails, but the rails have no bonding conductor to the earth bar and there is no DC surge protective device. Two DC extension leads cross the sharp edge of a rail without a sleeve, and the junction box on the last module of string 1 shows rust and a damp gasket.
1. State the safe preparation and test procedure for this insulation measurement, including why the inverter must be isolated and which test voltage to use.
Read the worked answer
This is a live-to-dead transition on a 454 V DC array, so sequence matters.
Preparation.
- Notify and lock off; the array cannot be switched off by the inverter, so treat all DC as permanently live while there is light. Work early morning, late evening or with the modules covered.
- Open the inverter DC switch and the DC isolator, then physically pull both string fuses or open both poles of the string connectors. Confirm dead with a CAT III meter rated for at least 600 V DC, testing phase-to-phase and each pole to earth.
- Disconnect the inverter's DC input cables. A 1000 V insulation test will destroy the inverter's input filter capacitors and its own insulation-monitoring circuit; the inverter must never be meggered.
- Disconnect any SPD from the circuit under test, since it will conduct at the test voltage and give a false low reading.
- Wear insulating gloves, use one-hand technique, and keep the array frames and rails treated as bonded metal only after you have verified the bond.
Test voltage. The array Voc is 454 V, so use 1000 V DC for a 1000 V class system. Test each pole to earth and to the frame, and short the string ends together first if you want a single pole-to-earth reading.
Acceptance. The inverter needs at least 1 megaohm to synchronise; healthy dry strings typically read well above 20 megaohm. Record the actual values, the test voltage, ambient conditions and time of day, because a wet reading must be repeated dry before you condemn anything.
2. Explain whether the missing frame bonding actually matters for a transformerless inverter, and specify the remedial earthing work.
Read the worked answer
It matters more than for an isolated inverter, not less.
Why. A transformerless inverter has no galvanic separation between the DC array and the AC supply. If a DC conductor faults to the frame and the frame is not bonded, the frame can sit at a potential derived from the AC side, and the leakage current that the inverter's insulation monitor is trying to detect has no defined return path. Bonding the array frame and rails to the installation earth bar does three things: it makes the frames safe to touch, it gives the insulation monitoring a stable reference so a genuine 0.4 megaohm fault is not confused with surface moisture, and it protects the DC side during induced surges, which is significant on a rooftop during the monsoon.
Remedial work.
- Bond every rail run and every module frame to the earth bar with a copper conductor of at least 6 mm2 (and no smaller than the DC conductor), in a continuous run with no daisy-chained single screws.
- Use bonding hardware that bites through the anodised aluminium, such as star washers or toothed bonding clips. Clamping alone is not a reliable electrical bond, because the anodising is an insulator.
- Fit a labelled DC earth bar at the array entry and connect it to the main earthing terminal with a verified resistance; the existing 1.8 ohm electrode is acceptable but check continuity of the whole path, not just the rod.
- Add a DC surge protective device at the array entry and an AC SPD at the inverter, both with short, straight leads to the earth bar.
- On the roof, re-route the two extension leads clear of the rail edge, sleeve them, and replace the damp junction box gasket and any corroded terminals.
- Record insulation resistance again dry, then wet, and file both readings.