TT-supplied workshop: loop impedance, RCD fault protection, and a proposed second earth rod
A metal-fabrication workshop on an industrial estate takes 400/230 V from the estate's own 11 kV/400 V transformer. The supply arrangement is TT: the workshop has a single earth electrode at the main switchboard, measured at 42 ohms by fall-of-potential. The main board contains a 63 A isolator, a 100 mA S-type RCD, and a 32 A type C MCB feeding a submain to a compressor and a spot welder. At the far end of that submain a loop-impedance tester records 46 ohms line-to-earth. The owner now wants a new 16 A circuit with six 13 A socket outlets for portable tools, taken from the same submain. He argues that the MCB will trip if anything goes wrong and that the 100 mA RCD is a backup anyway. A 30 mA RCD protecting the office sockets was tested and operated at 24 mA in 38 ms.
1. Show by calculation whether the 32 A type C MCB provides fault protection for the submain, and state the consequence for the new socket circuit.
Read the worked answer
MCB check. A type C device trips magnetically somewhere between 5 and 10 times its rating, so the required fault current is 160 A to 320 A. The maximum permissible loop impedance is therefore:
- best case (5 x In): Zs = 230 / 160 = 1.44 ohm
- worst case (10 x In): Zs = 230 / 320 = 0.72 ohm
Actual Zs is 46 ohms, giving a prospective fault current of 230 / 46 = about 5 A. That is only 0.16 x In, far below the magnetic threshold, so the MCB will not open instantly; the thermal element may take minutes or hours, if it operates at all. The MCB does not provide fault protection.
Touch voltage. While the fault persists, the rise on exposed metalwork is roughly If x Ra = 5 A x 42 ohms = 210 V, far above the 50 V touch-voltage limit. Disconnection must therefore come from the RCD, quickly.
Consequence for the sockets. In a TT system the RCD is the fault-protection device and must satisfy Idelta-n <= 50 / Ra. With Ra = 42 ohms that gives about 1.19 A, so the 100 mA device is acceptable for the fixed submain. However, a 16 A socket circuit supplying hand tools must have 30 mA protection (an individual RCBO or an extension of the existing 30 mA circuit), and the 30 mA device must be verified. Do not let the owner believe the MCB protects the sockets.
2. A colleague proposes driving a second earth rod so that Zs falls below 1.44 ohms and the MCB can be relied on. Evaluate that proposal and state what should actually be done.
Read the worked answer
The proposal confuses TN and TT practice. Tabulated maximum Zs values of the order of 1 ohm are written for TN systems, where the earth-fault path is a metallic return to the transformer star point. Here the return path is soil, and the measured 42 ohms is dominated by the electrode-to-soil resistance, not by conductor resistance. Reaching 1.44 ohms would require a large electrode field or a chemical/earth-mat installation; two rods in parallel typically only reduce resistance by 30-40 percent, never by a factor of thirty.
The correct engineering position for TT is:
- Accept that the MCB only provides overload and short-circuit protection, and that the RCD provides fault (earth-fault) protection using the 50 V rule.
- Verify the RCD instead: the 30 mA device must operate at <= 30 mA and within 300 ms at rated residual current, and within 40 ms at five times that current. The recorded 24 mA in 38 ms is a pass, but it should be re-tested in situ with the connected load.
- Confirm main protective bonding of all extraneous-conductive-parts (water, gas, steel structure, trunking) to the main earthing terminal, since bonding, not a low Ra, is what stops a potential difference appearing inside the building.
- Label the installation as TT and record Ra so future work does not repeat the assumption.
Improving the electrode is good practice for stability and for reducing the touch voltage during the RCD's operating time, but it is not the route to MCB disconnection.
3. List the verifications you would complete before the new socket circuit is energised, and the reason for each.
Read the worked answer
Before energising, complete and record:
- Protective conductor continuity and polarity. Verify the CPC of the new circuit and confirm that line, neutral and CPC are not transposed at any point; a reverse polarity at a socket is invisible to the user but lethal on a switched appliance.
- Insulation resistance. Test each conductor to the others and to earth at 500 V DC with the circuit isolated and sensitive equipment removed. A result of at least 1 megohm is the pass mark; expect hundreds of megohms on a new circuit.
- Earth-fault loop impedance (Zs) and RCD verification. Record Zs at the furthest socket to confirm the circuit conductors and bonding are sound, and test the 30 mA device with the instrument (trip current and trip time) as installed, not on the bench.
- RCD selection and coordination. Confirm the 30 mA device is upstream of the socket circuit and is selective with the 100 mA S-type at the origin so a socket fault does not trip the whole workshop.
- Bonding integrity. Re-check the main equipotential bonding connections disturbed during the work, and the CPC size relative to the line conductor.
- Documentation and labelling. Update the schedule of test results and the board chart, and note that the installation is TT, so no one later assumes the MCB clears earth faults.
Only after all of these pass should the circuit be energised and the sockets functionally tested with a plug-in tester.