- In this Q&A video, I'm going to be asking the question,
"Where do we need to use double pole protective devices?"
And we're going to illustrate the points
using some of these products from Lewden Palazzoli.
And to make sure you don't get caught out
when replacing a consumer unit,
check out our free training package on the subject
by clicking on the link in the subscription below.
Hi, I'm James Curnow,
and to find the answers to this question,
I'm going to be looking in BS 7671,
and there are a few chapters to look at to find the answer.
Now, there are some specific installations
that require double pole protective devices,
and they're in Part 7, "Special Locations,"
and they're as follows.
The first example is marinas and similar locations.
Regulations 709.531.2 says that,
"Socket-outlets shall be protected individually
by an RCD having the characteristics specified in 415.1.1.
Devices selected shall disconnect all poles,
including the neutral."
Also, "Final circuits intended for fixed connection
for the supply of houseboats shall be protected individually
by RCD having the characteristics specified in 415.1.1.
Devices selected shall disconnect all poles,
including the neutral."
The next example is solar,
for example, say, PV supply systems.
Regulations 712.432.101 says that,
"Where overcurrent protective devices are required
on the DC side,
both polarities shall be protected."
In outdoor lighting installations,
regulation 714.537.2.1.1 states that,
"Every circuit shall be capable
of being isolated individually
from each of the live supply conductors,
except as detailed in Regulation 461.2."
Now I'll talk about this Regulation
a little bit later in the video.
(birds chirping)
In caravans, Regulation 721.415.1 states that,
"Where protection by automatic disconnection
of supply is used, an RCD with a rated operating current
not exceeding 30 mA, breaking all live conductors,
shall be provided having the characteristics specified
in 415.1.1."
Also, Regulation 721.43.1 states that,
"Each final circuit shall be protected
by an overcurrent protective device,
which disconnects all live conductors of that circuit."
The next example is electric vehicles.
Joe Robinson has previously made a video about
double pole RCBOs for EV chargers.
So if you haven't seen that video already,
please watch that video after this one.
Next, we come to "Onshore Units
of Electrical Shore Connections
for Inland Navigational Vessels."
Regulation 730.531.3 says that,
"Socket-outlets with a rated current
not exceeding 63 A shall be individually protected
by an RCD providing additional protection
in accordance with Regulation 415.1
having a rated current not exceeding 30 mA.
The RCD selected shall disconnect all live conductors
i.e. line and neutral.
Note that this relates to the shore supply
rather than any onboard circuits,
which are outside of the scope of BS 7671."
The next example in Part 7
is temporary electrical installations
for structures, amusement devices,
and booths at fairgrounds, amusement parks, and circuses.
Regulations 740.537.2.2 says that "a device
for isolation shall disconnect all live conductors,
brackets, line and neutral conductors."
As I mentioned in a previous video here
on the eFIXX channel, if you're working
in an environment that is a special location in BS 7671,
I recommend checking Part 7
for any specific requirements.
The next chapter that relates
to this question is chapter 46, "Isolation Switching."
Regulation 461.2 states
that "In TN-C-S and TN-S systems, isolation
or switching of the neutral conductor is not required
if protective equipotential bonding is installed
and either, one, the neutral is connected to Earth
by a low resistance to meet the disconnection times
of the protective devices according to the requirements
of chapter 41 or the distributor declares
that either the PEN or neutral conductor
of the supply is reliably connected to Earth
by a low resistance to meet the disconnection times
of the protective devices according
to the requirements of Chapter 41."
However, if we look at regulation 462.2,
it says that "Every circuit shall be provided
with isolation means for all live conductors,
except as detailed in regulation 461.2,"
which is the regulation I mentioned a moment ago.
And that regulation relates to TN-C-S and TN-S systems.
So, in other words, if the system is a TT or an IT system,
we would need to be providing switching
for all live conductors.
And when we say live conductors, that includes the neutral,
which is also defined as a live part.
If we look in part two, "Definitions."
So, in a TT system,
if we intend on using the protective device for isolation,
we need to provide double pole protective devices.
However, regulation 462.2 also says
that "Provision may be made for a group of circuits
by common means, if the service condition allows this."
Now, what this means is,
we can use the double pole main switch
as the means of isolation.
However, we need to bear in mind that
if any of the protective devices are intended
to be used as a means of isolation,
then we would need to use a double pole protective device.
So, to give an example, if we look at table 534.7,
which is on page 184 of BS 7671,
the table lists types of protective and switching devices
and shows which devices are suitable for isolation.
And this shows that MCBs and RCBOs are suitable
for isolation.
However, note four refers to regulation 462.2,
which says that "every circuit shall be provided
with isolation means for all live conductors except
as detailed in regulation 461.2."
So, to summarize, if you're working on a TT system,
and if the protective devices are intended to be used
as a means of isolating circuits individually,
then double pole protective devices will be required.
I also mentioned IT systems.
Now, if you're wondering what I mean by IT system,
an IT system is a system which is insulated from Earth via
a sufficiently high impedance.
IT systems are used in situations
where a more resilient power supply is needed,
especially if loss of power would be dangerous.
And the most obvious example would be an operating theater
and other groups who've medical locations listed
in section 710.
Now, where we talk about double pole protective devices
that obviously relates to single face circuits,
but there are also requirements
for protecting the neutral that relate
to three face circuits.
So, for that, there are four pole protective devices
for situations where we need to protect the
neutral conductor as well as the line conductors.
For more information on this topic,
please also see the previous video that I mentioned earlier.
Chapter 43 in BS 7671 provides requirements
for the protection of live conductors
from the effects of overcurrent and regulation 431.2 relates
to protection of the neutral conductor.
Regulation 431.2.1 relates to TN or TT systems,
and says that "The neutral conductor shall be
protected against short-circuit current.
Where the cross-sectional area of the neutral conductor is
at least equivalent to that of the line conductors,
and the current is not expected to exceed the value
in the line conductors,
it's not necessary to provide detection
for the neutral conductor
or a disconnecting device for that conductor."
However, if we turn the page,
the regulation continues, "Where the cross-sectional area
of the neutral conductor is less
than that of the line conductors,
it is necessary to provide
overcurrent detection for the neutral conductor,
appropriate to the cross-sectional area of the conductor."
So a reduced size neutral is an example
of where we would need to provide protection
to the neutral conductor.
This would apply to three phase installations.
So, if you have ever wondered
whether you should use a three pole
or a four pole protective device
in a three-phase four-wire system,
a reduced size neutral would be one possible reason
for using a four pole device to make sure
that the neutral is protected against short circuit.
The next regulation is 431.2.2,
which relates to IT systems
and says, "The neutral conductor shall not be distributed
unless one of the following is met.
First, overcurrent detection is provided
for the neutral conductor of every circuit.
The overcurrent detection shall cause the disconnection
of all the live conductors
of the corresponding circuit, including the neutral.
Second, the particular neutral is effectively protected
against short-circuit
by a protective device on the supply side.
Third, the particular circuit is protected by an RCD
with a rated residual operating current, I delta n,
not exceeding the current-carrying capacity
of the correspondent neutral conductor."
So an IT system is another example of where
we would need to consider protecting the neutral conductor.
However, in regulation 411.6.1, there is a note that says
that "It is strongly recommended that IT systems
with distributed neutrals should not be employed."
The next regulation is 431.2.3,
which relates to harmonic currents.
Again, this relates to three phase systems,
and the regulations says that
"Overcurrent detection shall be provided
for the neutral conductor in a polyphase circuit
where the harmonic content of the line conductors is such
that the current in the neutral may exceed
the current-carrying capacity of that conductor."
So, in summary, there are various examples
of where we need to consider double pole protection
for single phase circuits
and four pole protection devices for three phase circuits.
If you haven't seen Joe's previous video on double pole RCDs
for EV chargers, please see that video after this one.
Thanks for watching.