What earth resistance do I need for a TT system?

For a TT system protected by a 30mA RCD (the most common arrangement in the UK), the earth electrode resistance must not exceed 1667 ohms (calculated as 50V ÷ 0.03A). However, a maximum of 200 ohms is widely recommended as good practice to ensure reliable RCD operation and provide adequate fault current. BS 7671 Regulation 411.5.3 requires that Ra × Ia ≤ 50V, where Ra is the total earth fault loop impedance and Ia is the RCD operating current.

How do I test earth electrode resistance?

Earth electrode resistance is tested using a dedicated earth electrode tester (e.g., Megger DET4) with the fall-of-potential method. Two temporary test spikes are driven into the ground in a straight line from the electrode under test. The potential spike is placed at 61.8% of the distance to the current spike (the 61.8% rule). At least three readings should be taken at different spike positions to verify consistency. Testing should be done in dry conditions for worst-case results.

Does the time of year affect earth resistance?

Yes, earth electrode resistance varies seasonally. It is lowest in winter when the ground is wet and highest in late summer when the soil is driest. Resistance can vary by a factor of 2-3 between wet and dry seasons. For this reason, testing in dry conditions gives the most conservative (worst-case) result. If tested in winter, apply a seasonal correction factor — typically 1.5 to 2.0 for UK conditions.

What type of earth electrode should I use?

The most common earth electrode in the UK is a driven copper-clad steel rod, typically 1.2m sections that can be coupled together and driven to 2.4m or deeper. Earth rods should be at least 16mm diameter for copper-clad steel. Alternatives include earth plates (minimum 0.5m², 2mm thick copper), earth tapes (minimum 25mm × 2mm copper strip), and foundation earth electrodes. The choice depends on soil conditions and the target resistance.

Earth Electrode Resistance — TT System Reference

Target earth electrode resistance values for TT earthing systems, soil resistivity by type, testing methods and seasonal variation guidance per BS 7671.

Target Earth Electrode Resistance Values

RCD Rating (mA)	Max Ra for 50V (ohms)	Recommended Max (ohms)
30	1667	200
100	500	200
300	167	100
500	100	50

Ra × Ia must not exceed 50V per BS 7671 Regulation 411.5.3 (TT systems).

Soil Resistivity by Type

Soil resistivity directly affects earth electrode resistance. Lower resistivity soils give lower electrode resistance.

Soil Type	Resistivity (ohm-metres)	Typical Electrode Ra*
Marshy ground, peat	5 – 40	2 – 15
Clay (wet)	8 – 50	3 – 20
Clay (dry)	50 – 150	20 – 60
Loam, garden soil	50 – 200	20 – 80
Sand (wet)	100 – 300	40 – 120
Sand (dry)	1000 – 3000	400 – 1200
Gravel	300 – 1000	120 – 400
Chalk	100 – 500	40 – 200
Limestone	500 – 10,000	200 – 4000
Rock (granite, slate)	1000 – 100,000	400+

* Typical Ra for a single 2.4m driven rod. Multiple rods or longer rods will reduce resistance.

Methods to Reduce Earth Resistance

Drive rods deeper — doubling length reduces resistance by approximately 40%
Use multiple rods in parallel — spaced at least their depth apart (e.g., 2.4m rods at 2.4m spacing)
Use earth enhancement compounds (e.g., bentonite, Marconite) around the electrode
Install earth plates or earth mats for very high resistivity soils
Locate electrodes in the wettest available ground