Fault Clearing Time and Grid Safety

Short answer. The body tolerates more current the shorter the shock lasts. That is why, in IEEE Std 80-2013, the time the protection takes to clear the ground fault enters directly into the calculation of the tolerable step and touch voltages: the tolerable current is proportional to 1/√t. Faster clearing gives a higher tolerable voltage; slower clearing requires a grid that produces lower voltages.

Why time counts twice

Clause 5.3 explains that fast clearing helps for two reasons. The first is that it reduces the probability of exposure to shock, in contrast with situations in which the current would persist for minutes or hours. The second is that tests and experience show that the risk of serious injury or death drops if the body current lasts a very short time.

In the calculation, time appears in the Dalziel formula: the tolerable current is k/√t, with k equal to 0.116 for a 50 kg person and 0.157 for 70 kg. With the data of the guide’s Annex B example (400 Ω·m soil, 2500 Ω·m gravel with Cs = 0.74, 70 kg person), the tolerable touch voltage changes as follows:

The 0.25 s and 1.0 s values result from applying the same formula to the example. Dividing the time by 4 doubles the tolerable voltage.

Which time to take

The guide does not impose a value. It poses a judgment decision between two times:

• Primary protection. It is defended by the low combined probability that a relay fails just when all the other adverse factors of an accident coincide.
• Backup protection. It is more conservative and gives a larger safety margin.

According to clause 16.2.2, the fault time and the shock time are assumed equal, unless the duration is the sum of successive shocks, such as those of reclosing. The choice must give the most pessimistic combination between the decrement factor of the fault current and the current the body can withstand. The typical values of both times are between 0.25 s and 1.0 s, and the guide suggests fast times for transmission substations and slower ones for distribution and industrial substations.

The clearing time is the sum of the relay operation and the opening of the circuit breaker. For the second part see circuit breaker opening time and interrupting time.

Reclosing and successive shocks

A person can withstand a first shock without permanent injury. An instantaneous automatic reclose can produce a second shock less than 0.33 s after the start of the first, and that second one can cause a serious accident. With manual reclosing the risk is lower because the interval is usually longer.

The cumulative effect of very close shocks is not fully evaluated. A reasonable allowance, adopted by the guide, is to add the durations of each shock as if it were a single one. For how reclosing works, see automatic reclosing 79.

What fast clearing gains

Clause 16.6 includes among the ways to raise the tolerable voltages reducing the clearing time, using a more resistive surface layer or increasing its thickness.

The guide also warns that limiting the fault current at the cost of longer clearing times can make things worse, because the body tolerates less current as time grows. Backup times, on the other hand, are the ones usually used to size the grid conductor: in small substations they can approach 3 s or more, whereas in large ones, with complex or redundant protection, the fault is generally cleared in 1 s or less.

How it relates to your substation’s protection

If the clearing time changes, grid safety changes with it. A slower protection setting, a backup relay with a longer delay or a breaker failure that forces waiting for the backup protection lengthen the shock and reduce the tolerable voltage. That is why the grid should be reviewed when settings are modified, when current sources that raise the fault are added or when the protection scheme changes. See also breaker failure protection 50BF and what a protection coordination study needs.

What protection data the grid design needs

To set the shock time, the following data must be gathered for each zone of the substation:

• The clearing time of the primary protection and that of the backup.
• The opening time of the circuit breaker that clears the fault.
• If there is reclosing, how many attempts it makes and with what delay.
• The expected changes in the fault current sources.

With them the time is chosen and it is checked that the grid voltages remain below the tolerable ones. The maximum potential the grid reaches with that current is calculated as explained in ground potential rise (GPR).

We carry out your grounding study

We define with you which clearing time to adopt in each zone of your substation and calculate the tolerable voltages with it. In the “Tolerable” tab of our free grounding resistance tool you can see how the touch and step limits change with the shock duration. See the engineering design and studies service or write to us through contact. If you also need to adjust your distance and overcurrent protection, we can also configure your relays remotely.

Related guides

• IEEE 80: Safety in Substation Grounding
• Tolerable Body Current According to IEEE 80
• Ground Potential Rise (GPR): What It Is and How to Calculate It
• Automatic reclosing 79 (RREC): what it is and how it works
• Breaker failure 50BF (RBRF): what it is and how it works

References

• IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 5.3 (importance of fast clearing), 6.1 and 6.2 (duration formula and choice of time), 6.4 (reclosing), 11.3.3 (additional factors for sizing the conductor, including backup time) and 16.2.2 (fault and shock duration); equations (7) to (9) and Annex B, example 1.

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