There are a couple of questions on the standard Installation form for
which I would appreciate some help.
1. Prospective fault current
2. External loop impedance
How do I calculate or obtain these figures ?
Are there defaults that can be assumbed and used ?
(John, many thanks for the email)
External Loop impedence (Ze) is obtained from the electricity
supplier. It is the impedence of the circuit loop from the
consumer back to the supply transformer and then
back to the consumer.
You have to pester your local leccy company and try and find
somebody who knows what "what's the Ze here?" means. I
wrote it down in my On-Site Guide as soon as I found it.
It's 0.35Ohm in Sheffield.
Calculating prospective fault current in in the regs, section 3.7
On Fri, 4 Jan 2008 06:44:03 -0800 (PST) someone who may be ac1951
The two are related, as the impedance determines what the
prospective fault current is.
Option 1. Measure the supply and calculate. How this is measured
depends on the equipment one has available, or hires. A fair
indication can be obtained by using a basic moving coil meter to
measure how the supply voltage varies with known loads. Don't try
this unless you know what you are doing.
Option 2. You can ask the supplier and they may eventually give you
a figure. This is likely to be the worst case, in order to protect
their backsides. I would ignore any figures they might provide.
Ze really needs to be tested on site. It is perfectly possible (although
rare) for there to be a fault in the supply. The supplier will only quote
the maximum allowed Ze for the type of supply. I saw two Ze values last year
that were out of spec and that was from of hundreds of tests.
Out via the line (phase) conductor and back via whatever is providing
the earth, that is.
Only if you have a PME (TN-C-S) supply, where the earth path is via the
neutral conductors of the service cable and street mains.
For TN-S supplies, where earthing is via the sheaths or armouring of
cables, the default quoted value is 0.8 ohm.
For TT supplies, where the supplier doesn't provide a metallic earth
path, the value is 21 ohms (20 ohms being the statutory limit on the
DNO's earthing electrodes, plus a notional 1 ohm for the metallic
These are national maximum values which apply for
single-phase supplies of up to 100 A, but they are not guaranteed.
When testing your installation you should measure and record Ze using a
loop tester (for TN systems only). To do this the main bonding should
be temporarily disconnected from the main earth terminal, so that the
result is not artificially lowered by the effect of any fortuitous
parallel earth paths. If a value significantly above 0.35/0.8 ohm is
found the DNO should be notified.
You can only calculate the PSSC from a Ze value for the PME case, where
the earth and neutral paths are one and the same. For the other systems
the supply impedance between line and neutral is usually lower than Ze,
so the PSSC would be underestimated if calculated from Ze.
The calculation is Ipssc = Uoc / Z, where Uoc is the o/c phase voltage
at the supply transformer (usually taken as 250 V) and Z is the measured
impedance at the supply terminals. Most modern loop testers will do the
calculation for you, using a voltage value measured prior to the test.
For certification purposes though it's OK to just fill in the DNO's
declared maximum value, which will usually be 16 kA (again for 1-ph, 100 A).
These values can me measured/calculated for an individual property.
The "standard" answer you will get from the regional electricity
distribution company is based on the "standard" values in this document:
"EA Engineering Recommendation P25/1" (The short-circuit characteristics
of Public Electricity Suppliers? low voltage distribution networks and
the co-ordination of overcurrent protective devices on 230V
single-phase supplies up to 100A), which is not available for free
However, the relevant info is available from vairious websites, e.g.
is summarised here:
Low Maximum prospective short circuit current is used to determine the
short circuit rating requirement of electrical equipment.
Low Voltage Connections (up to 1000V)
The following values can be assumed at low voltage connections: * Single phase connection; 16kA
* Two phase, split phase or three phase connection; 25kA
The maximum earth fault loop impedance is used to calculate the
available earth fault current within an installation connected at low
voltage (below 1000V).
The following values are typical for low voltage installations:
Earthing Type Maximum Earth Fault Loop Impedance
PMEarthing (TN-C-S) 0.35 ohms*
TN-S 0.8 ohms*
Direct Earth (TT) Dependent on resistance of the customer?s earth
electrode. Values exceeding 100 ohms can apply.
* These values are sometimes exceeded where the connection is derived
from a long overhead line network or from a small pole mounted transformer.
In practice, a 16kA PSCC would only apply if you were very close to a
My last house was about 200m from a small sub-station and featured a
PSCC of 750A. You can get a higher PSCC from a 100kVA generator!
The BS1361 type II suppliers cut-out fuse protecting the property is
rated at 33kA breaking capacity.