In short. Temporary sites are where earthing is hardest to keep intact: cables are dragged and driven over, boards are moved, electrodes are driven in whatever ground is available, and the people near the equipment are often working in wet conditions or are members of the public. The answer is a deliberate arrangement: the rental generator’s neutral earthed at the set, a protective conductor run to every board wherever possible, a local electrode at each board, and residual current devices (RCDs) chosen circuit by circuit, with the touch-voltage limit taken as 25 V instead of the usual 50 V. Reddy Generators rents generators from its own fleet across Hyderabad, Telangana and Andhra Pradesh; this page sets out how the earthing and RCD protection for such a hire should be planned and checked.
Who this page is for: Site engineers, event organisers, project managers and facility teams in Hyderabad, Telangana and Andhra Pradesh who are setting up temporary power from a rental generator and need the earthing and residual current protection to be safe from the first day.
Key facts
- On construction sites, temporary installations and agricultural premises, many codes and guides apply a reduced touch-voltage limit of 25 V instead of 50 V, and this page uses it for every TT board, so RA × IΔn ≤ 25 V.
- Socket outlets up to 32 A and hand-held tools on construction sites are protected by RCDs of 30 mA or less in international practice (IEC 60364-7-704).
- The CEA safety regulations of 2023 (Regulation 44) require electrical installations to be controlled by a residual current device, with an operating current not exceeding 30 mA for domestic installations.
- Upstream RCDs must be time-delayed (S type or adjustable) and have a higher rated residual current (lower sensitivity) than downstream ones, or one fault blacks out the whole site.
- Drives, EV chargers and electronics need the right RCD type (A, F or B). A type AC device can be blinded by DC leakage and fail to trip.
The parent page Earthing and Neutral Grounding explains in a paragraph why a rental set must have its neutral earthed and its frame bonded. This page builds on that with the arrangement, the numbers and the checks for real temporary sites. Permanent generator earthing is covered in DG Set Earthing, Step by Step.
The two arrangements you will meet on site
| Arrangement | How it is built | What clears an earth fault | Where it fits |
|---|---|---|---|
| TN-S from the set | Neutral earthed and bonded to the frame at the set; every cable carries a protective conductor (a separate core, or armour proven adequate) to every board; local electrodes at boards bonded to that conductor | The metallic loop back to the alternator, with RCDs as additional protection | Most sites where all boards are fed with 5-core or 4-core-plus-earth cable |
| TT at a board | The board’s metalwork and outgoing protective conductors are earthed only to a local electrode; no protective conductor runs back to the set | Only an RCD, because the fault current through two electrodes in series is far too small for a breaker | Remote boards fed by cable without a usable protective conductor; labour camps; some event sites |
Running a protective conductor to every board is the stronger arrangement and should be the default. Where a board is TT, the 25 V rule below is not optional: it is the only thing that makes the board safe.
The 25 V rule: maximum electrode resistance for each RCD
RA is the resistance of the board’s electrode plus its protective conductor, at its worst over the hire. If you measure in the monsoon and the hire runs into the dry season, divide the allowed value by about 3.
| RCD rated residual current | Maximum RA at 25 V | Same, if measured in the wet season (÷ 3) | For comparison: 50 V (permanent installations) |
|---|---|---|---|
| 30 mA | 833 Ω | 278 Ω | 1,667 Ω |
| 100 mA | 250 Ω | 83 Ω | 500 Ω |
| 300 mA | 83 Ω | 28 Ω | 167 Ω |
| 500 mA | 50 Ω | 17 Ω | 100 Ω |
| 1 A | 25 Ω | 8 Ω | 50 Ω |
A 30 mA RCD tolerates a very poor electrode; a 1 A incomer needs a good one. That is why the sensitive devices go at the final circuits, and why a high-rated, time-delayed RCD at the incomer has to be checked against the electrode it actually relies on.
Choosing RCD ratings, circuit by circuit
| Circuit | Typical choice | Reason |
|---|---|---|
| Socket outlets up to 32 A, hand tools, portable lighting | 30 mA, instantaneous (general type) | Additional protection against direct contact; the person holding the tool is the one at risk |
| Event circuits accessible to the public (stalls, stage, lighting on stands) | 30 mA, instantaneous | Wet ground, crowds, no control over who touches what |
| Fixed equipment above 32 A (tower crane, hoist, batching plant) | 100–300 mA, type chosen for the load, often time-delayed | Normal leakage of large equipment would trip 30 mA devices |
| Sub-main feeding a board of 30 mA circuits | 300 mA, S type | Protects the cable and board; stays in while a 30 mA device clears its own fault |
| Main incomer at the generator | 300 mA to 1 A, time-delayed (S type or adjustable) | Fire protection and backup; must be selective with everything below |
Selectivity. For one RCD to stay closed while the one below clears a fault, two things are needed: the upstream device’s rated residual current should be at least about three times the downstream one’s (a 300 mA S type above 30 mA devices is the classic pair), and the upstream device must be time-delayed. General-type RCDs typically trip within 0.3 s at their rated current and 0.04 s at five times it; S-type devices have a short built-in non-operating time (around 0.13 s at rated current) and must trip within about 0.5 s. Two S-type devices in series are not selective with each other; use an adjustable time delay at the incomer.
Standing leakage. An RCD may trip at anywhere from half its rated current upwards. Keep the normal leakage of the equipment on each RCD below about 30 % of its rating: no more than about 9 mA on a 30 mA device. Filters in drives, LED screens, UPS units and IT equipment each leak a few milliamps, and together they cause the “RCD keeps tripping” calls.
RCD types for modern loads
| Type | Detects | Use for |
|---|---|---|
| AC | Sinusoidal AC residual current only | Plain resistive loads; increasingly unsuitable on sites full of electronics |
| A | AC plus pulsating DC | General electronics, single-phase power supplies, LED lighting and screens, most site cabins |
| F | As type A, plus mixed-frequency residual current from single-phase frequency converters | Single-phase inverter air-conditioners, single-phase drives, pumps with built-in converters |
| B | As type F, plus smooth DC and currents up to about 1 kHz | Three-phase variable-speed drives (tower cranes, hoists, pumps), EV chargers without built-in DC fault detection, PV inverters |
Each type also meets the requirements of the types above it in the table. A type AC or A device upstream of a three-phase drive can be desensitised by DC leakage and fail to trip for a real fault; that is a hidden failure, not a nuisance. EV chargers usually need either a type B RCD or a type A RCD with the charger’s own 6 mA DC detection, as the charger maker specifies.
Site types: what changes
| Site | Main earthing concern | Practical arrangement |
|---|---|---|
| Construction | Damaged cables, moving boards, crane and hoist drives, welding, wet work | TN-S to every board with local electrodes; 30 mA on all sockets; type B for crane and hoist drives; welding sets on their own circuits with the work return lead clamped close to the weld |
| Events and exhibitions | Public access, wet grass, short set-up time, LED walls, catering equipment | Electrode at the set and at each distribution point; 30 mA on everything the public can reach; leakage budget for LED screens; cable ramps |
| Apartments during DG overhaul | Rental set connected into the building’s changeover for weeks | Rental set’s body and neutral connected to the building’s existing DG earthing system through its test links (after testing) or to new electrodes bonded to the main earthing bar; see Apartments during DG Overhaul |
| Bank and office branches | Set parked outside, cable into the building panel, often for months | Set earthed locally and bonded to the building earth; the building’s existing RCDs and changeover arrangement checked before connection |
| Shutdowns in plants | Set feeding parts of a permanent installation | Plant’s earthing system used, with the neutral-to-earth link located deliberately; permit-to-work for all connections |
Worked example: a construction site on a 125 kVA rental set
Illustrative case. A residential tower under construction on the outskirts of Hyderabad takes a 125 kVA rental set. The distribution is:
- Main board at the set; set earthed to two backfilled rods measuring 9 Ω combined.
- Board 1: tower crane on a three-phase drive, fed with 5-core cable, local electrode 12 Ω. The drive’s filter leaks about 25 mA in normal running.
- Board 2: site office and socket circuits, 5-core cable, local electrode bonded to the protective conductor.
- Board 3: labour camp 200 m away, fed with an existing 4-core cable with no usable protective conductor, so it is TT on its own electrode, which measured 64 Ω in August.
- Board 3 (TT), seasonal correction: the hire runs to May, so the design value is about 3 × 64 = 192 Ω.
- Board 3 incomer, first proposal 300 mA: 192 × 0.3 = 57.6 V. That exceeds 25 V in the dry season (it would have passed in August at 19.2 V). Fails.
- Board 3 incomer at 100 mA, S type: 192 × 0.1 = 19.2 V, which passes. The camp’s socket and lighting circuits get 30 mA devices: 192 × 0.03 = 5.76 V. Better still, a second electrode at the camp or a new cable with a protective conductor would remove the dependence on one rod.
- Board 1, crane: a 30 mA device would sit at 25 mA of standing leakage, above the half-rating point at which it may trip; it would trip randomly. A 300 mA type B, time-delayed RCD is used: leakage 25 mA is about 8 % of its rating, inside the 30 % budget, and 12 Ω × 0.3 = 3.6 V, well under 25 V even if the protective conductor were damaged.
- Board 2, offices: a 300 mA S type sub-main above 30 mA type A final circuits; 300 ÷ 30 = 10 times, so the pair is selective. Board 2’s metalwork is on the protective conductor from the set, so faults clear through the metallic loop as well.
- Main incomer at the set: an adjustable RCD set at 1 A with a 1 s delay (the longest normally allowed for distribution circuits), so it stays in while any 300 mA or 100 mA S type downstream clears, which they must do within about 0.5 s. Its touch-voltage check against the set electrode: 9 × 1 = 9 V, under 25 V.
- Result: one change from the first proposal (Board 3 incomer from 300 mA to 100 mA) made the difference between a board that was safe only in the monsoon and one that is safe all year.
Exceptions and things to watch
- Do not import a utility earth to a site set’s board by connecting the generator’s protective conductors to a nearby utility pole or a neighbour’s supply; it links two systems that may be at different potentials.
- Welding: return currents wander through any metalwork they can find, including protective conductors; clamp the return lead at the work and keep welding sets on their own circuits.
- Very rocky sites: where a low electrode value is unachievable, keep every board on a protective conductor from the set and rely on 30 mA devices for all final circuits; see the design calculation page.
- Safety services (fire pumps, emergency lighting) on temporary supplies should not sit behind sensitive RCDs that could disconnect them during an emergency; they need a designed arrangement.
- Small portable sets may be used without an earth electrode only where the maker states the set provides electrical separation and a single appliance is connected; otherwise, and whenever the set feeds a fixed installation, earthing and RCDs as above apply.
Pre-energisation checklist
- Set neutral earthed and bonded to the frame at the set (or the building’s link location confirmed where connected to a building).
- Set electrode installed, test link fitted, reading recorded with the date.
- Protective conductor continuous from the set to every board; continuity tested.
- Local electrode at each board, bonded; readings recorded; TT boards identified and labelled.
- RA × IΔn ≤ 25 V checked for every TT board using the dry-season value.
- RCD ratings and types match the table above; selectivity checked (rating ratio and time delay).
- Every RCD tested with an RCD tester for trip time at rated current and at five times rated current, not just the test button.
- Boards closed, lockable, weatherproof and raised off wet ground; cables routed, protected at crossings, no joints on the ground.
- Changeover interlocked so the set cannot back-feed the utility supply.
- Emergency stop location known to the site team.
Daily checks for the site team
- Press each RCD’s test button at the start of the day on construction sites and events; it must trip. Reset and log it.
- Walk the cable routes: no cuts, crushing, exposed cores or joints lying in water.
- Check that earth leads at the set and boards are attached and not disturbed by vehicles or excavation.
- Report any RCD that trips repeatedly; never bypass it or replace it with a higher rating to keep working.
- Keep the set’s surroundings dry and free of fuel spills around electrical connections.
| Item | Board 1 | Board 2 | Board 3 |
|---|---|---|---|
| Board / circuit | |||
| Earthing arrangement (TN-S or TT) | |||
| Electrode reading and date | |||
| RCD rating, type, delay | |||
| RA × IΔn (V) | |||
| Trip time at IΔn and 5 × IΔn | |||
| Checked by and date |
Common mistakes
- Feeding a remote board with cable that has no protective conductor and protecting it with a high-rated RCD that does not meet the 25 V rule for its electrode.
- Putting a tower crane or other drive on a 30 mA or type AC RCD, then replacing it with a bigger device when it trips.
- Fitting general-type RCDs at every level, so a single fault on one socket trips the whole site.
- Measuring the electrode once in the monsoon and assuming the same value through the dry season.
What to send us
Send the site address and type (construction, event, apartment overhaul, office), the hire dates, the main loads with any cranes, hoists, drives, welding sets, LED screens or EV chargers marked, a sketch showing where the set will stand and where each distribution board will be with approximate cable distances, and whether the set will feed boards directly or connect into a building’s changeover. With that we can propose the set and advise on the cabling, earthing and RCD arrangement for the hire. Use the generator rental enquiry, or WhatsApp the details and photographs.
Questions site teams ask
Does a rental generator need an earth pit?
Yes. The set’s neutral must be earthed and its frame bonded, and the set needs an earth electrode at its location. On longer hires the electrode should have a test link and a recorded reading, like a permanent installation.
What RCD rating should I use on a construction site?
30 mA for socket outlets up to 32 A, hand tools and portable lighting; 100–300 mA, time-delayed, for large fixed equipment and sub-mains; and a time-delayed device at the incomer that is selective with everything below it.
Why does the RCD keep tripping on our site generator?
Usually because too much equipment with normal leakage (drives, LED screens, UPS, computers) is on one RCD, or because of a real insulation fault in a damaged cable or wet equipment. Split the load, use the right type and rating for large drives, and find the fault before resetting repeatedly. Never bypass the RCD.
What earth resistance is acceptable for a temporary site board?
For a board earthed only to its own electrode, the electrode resistance times the RCD rating must not exceed 25 V. With a 30 mA RCD that allows up to about 833 Ω; with 300 mA only about 83 Ω. Use the dry-season value.
Which RCD type do I need for a tower crane or EV charger?
Three-phase variable-speed drives such as tower cranes and hoists usually need type B. EV chargers need type B, or type A where the charger has its own DC fault detection, as the charger maker specifies.
Can the rental generator use the building’s earth?
When the set is connected into the building through a changeover, it is usually connected to the building’s earthing system, with the neutral-to-earth link located as the building’s design requires. The connection and changeover must be made by competent, licensed people.
How often should RCDs on a site be tested?
Press the test button daily on construction sites and events, and test trip times with an RCD tester when the installation is set up, after any change, and periodically during long hires.
Related pages
- Earthing and Neutral Grounding
- DG Set Earthing, Step by Step
- Earth Resistance Testing
- Earthing Design Calculation
- Construction Site Generator Rental
- Event Generator Rental
- Apartments during DG Overhaul
- Earth-Fault and Ground-Fault Protection
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.