Substation earthing
HOT / COLD and high EPR site classification (ENA TS 41-24, EREC S34)
- where
- earth potential rise (V)
- ITU-based HOT site threshold (V)
- fault clearance time (s)
- permissible touch voltage, shoes on soil, from ENA TS 41-24 Table 1 (V)
Explain this equation
Sorts a site into HOT or COLD and high or low EPR, the two classifications UK network operators use to decide on telecoms protection, LV earthing arrangements and how much detailed assessment is needed.
UITU is the ITU-derived EPR limit for telecoms equipment: 650 V where protection clears faults in 0.2 s or less, and 430 V otherwise. UTp is the permissible touch voltage for a person in shoes on soil, read from ENA TS 41-24 Table 1 at the clearance time t. Twice UTp is the high EPR threshold.
The factor of 2 reflects the fact that a meshed grid normally limits touch voltage to around half of the EPR or less, so an EPR up to 2 UTp is accepted without a detailed touch voltage study. Above that the touch and step voltages must be worked out.
At 1 s clearance UTp on soil is 233 V, so the high EPR threshold is 466 V and the HOT threshold is 430 V. A typical 33 kV primary with 2 kV EPR is both HOT and high EPR.
Watch out: the HOT limit protects telecoms circuits; it is not a safety limit for people. Check the DNO's own policy, which may use different figures.
Classifies a site as HOT or COLD against the ITU-derived EPR limits used in UK practice, and as high or low EPR against twice the permissible touch voltage.
Enter your values to see the result.
Good to know
The site is HOT if EPR > U_ITU, where U_ITU = 650 V if the fault clears in 0.2 s or less and 430 V otherwise, and high EPR if EPR > 2 × UTp (UTp interpolated in ENA TS 41-24 Table 1). ENA TS 41-24 Issue 2 defines a HOT site as one whose EPR exceeds ITU limits, typically 650 V rms where faults clear in 0.2 s or less (high-reliability protection) and 430 V otherwise. It defines high EPR (HPR) as EPR greater than twice the permissible touch voltage; at 1 s on soil this is 2 × 233 V = 466 V, the figure used in EREC S34 Case study 5. BS EN 50522 clause 5.4.2 uses the same idea: EPR ≤ 2 UTp (condition C2), or ≤ 4 UTp with specified measures (C3). UTp here is interpolated linearly in TS 41-24 Table 1 (shoes on soil or outdoor concrete).
Common mistake
Treating the 430/650 V HOT limits as safety limits. They protect telecoms equipment and staff working on it; they are not touch-voltage limits. A COLD site can still have unsafe touch voltages and a HOT site can be safe.
Limits
Confirmed: TS 41-24 Issue 2 still quotes 430 V / 650 V for HOT sites and 2 × UTp for high EPR. Network operators may use their own figures (some apply 430 V regardless of clearance time; Openreach agreements may differ), so check the DNO policy and telecoms operator. UTp is for shoes on soil; use the chippings rows of TS 41-24 Table 1 where they apply. Valid for clearance times 0.1–10 s.
Sources
- ENA TS 41-24 Issue 2 Guidelines for the design, installation, testing and maintenance of main earthing systems in substations (consultation text, Tables 1 and 2)
- ENA EREC S34 Issue 2 (2018) A guide for assessing the rise of earth potential at electrical installations
- BS EN 50522 Earthing of power installations exceeding 1 kV a.c. (2010; superseded by EN 50522:2022)
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.