Wiring mishap?

[Andrew Gabriel]

70 years ago there was a type of wirenut which was ceramic and had a
molded-in conical thread, which did not work nearly as well as modern
wirenuts with a metal conical threaded insert. I am not sure where you
are that they are illegal, but in North America almost every building has
wirenuts in lighting wiring in multiple places, and all but perhaps a few
badly installed wirenuts have redundant, gas-tight connections both
between the wires and the conical threaded insert, and directly from wire
to wire, with each contact area being larger than the wire cross
section. Take one apart after 40 years or more and you will still see
bright copper at both contact areas, solid proof of a gas tight
connection.

In discussions of the detailed causes of electrical fires, I have never
heard any reference to wirenuts being a significant problem, nor did a
web search turn up anything, so as of now I see no factual basis for the
claim that there is any problem with wirenuts other than the possibility
of faulty installation by untrained amateurs - and that happens with all
wiring methods!

Well, no actually. A lot of work has taken place over the last ~60
years to make our wiring accessories safe and reliable when used by
untrained amateurs, which is a big market in the UK (which also
makes them much safer when used by professionals).
Safety and ease of use have been big selling points here for the
last 60 years, and manufacturers have continuously tried to
out-compete each other in this area, which results in continuous
product improvement. When I take a walk down the electrical isle
in the US (such as Home Depot), it's like stepping back 60+ years
to what we used to have before that.

Some 10+ years ago, I was in an Electrical Engineering meeting in
New York. That gave me the opportunity to ask one of the large
manufacturers why there's such a difference between the state of
UK and US wiring accessories, given that they manufacture products
for both countries. The answer was partly what I said above - buyers
will buy based on quality and safety and ease of use in the UK,
which is where manufacturers compete. In the US, buyers will only
buy on price, so whilst he could make a safer duplex outlet at 75c,
no one will buy it because there's a bin full of 50c ones there.
US Elecetrical Engineers know they have an electrical fire rate
which is well in excess of all other countries in similar economic
bracket, but there's no appetite to tackle the causes, merely to
paper over them with things like arc fault interrupters. Many other
countries do not need arc fault interrupters - you might like to
ask yourself why that might be...

Much of what I say for the UK applies to much (but not all) of the
EU too.

 

[Glen Walpert]

Well, no actually. A lot of work has taken place over the last ~60 years
to make our wiring accessories safe and reliable when used by untrained
amateurs, which is a big market in the UK (which also makes them much
safer when used by professionals).
Safety and ease of use have been big selling points here for the last 60
years, and manufacturers have continuously tried to out-compete each
other in this area, which results in continuous product improvement.
When I take a walk down the electrical isle in the US (such as Home
Depot), it's like stepping back 60+ years to what we used to have before
that.

Some 10+ years ago, I was in an Electrical Engineering meeting in New
York. That gave me the opportunity to ask one of the large manufacturers
why there's such a difference between the state of UK and US wiring
accessories, given that they manufacture products for both countries.
The answer was partly what I said above - buyers will buy based on
quality and safety and ease of use in the UK, which is where
manufacturers compete. In the US, buyers will only buy on price, so
whilst he could make a safer duplex outlet at 75c, no one will buy it
because there's a bin full of 50c ones there.
US Elecetrical Engineers know they have an electrical fire rate which is
well in excess of all other countries in similar economic bracket, but
there's no appetite to tackle the causes, merely to paper over them with
things like arc fault interrupters. Many other countries do not need arc
fault interrupters - you might like to ask yourself why that might be...

Much of what I say for the UK applies to much (but not all) of the EU
too.

You make some good points, and indeed there is a lot of low quality junk
at the Home Despot which is usually what goes into new construction
here. But you can also get good quality parts; even the Despot has good
quality $3 "spec grade" receptacles next to the $.50 junk. Likewise with
pretty much all other parts of the electrical distribution system. Most
commercial and industrial power distribution here is of good quality, and
some few (like myself) have all commercial grade electrical components in
their home. (I replaced every receptacle and switch in the house,
inspected and torqued every connection, and generally replaced anything
not up to my standards.) But the lowly twist-on connector or wirenut
does not seem to be a real part of this problem, other than also being
available in low and high quality versions.

I would provide my explanation of why we have such crappy electrical
parts in use here, but this is not a politics newsgroup .

Regards,
Glen

 

[[email protected]]

Yes. The connectors are rated in cross-sectional conductor area,
and you can put in as many conductors as you like, totaling up to
that cross-sectional conductor area.

Makes sense. I knew they had a maximum wire gauge but I didn't know it
was OK to just add them up. (This is a lot easier to figure out when
your wires are known by their cross section in real units, rather than
by gauge.)

Stranded flex should have a bootlace ferule applied unless the
chocolate block connector is specifically designed for flex (in which
case either the grub screw end is matched to the curvature of the
bottom of the channel, or it has a metal leaf protection spring under
the grub screw).

Ah, *that's* what that piece of metal is for. I had seen it before,
often on terminals that are used to connect small-ish (<0.8 mm^2 or so)
loose wires to a circuit board, but didn't quite understand why it was
there.

The general principle of connections is that the contact pressure
should be high enough to be gas-tight (which prevents oxidation). That
can't be achieved with a wirenut; it needs a screw clamp or a crimp.

The "chocolate block" connectors also seem less scary to work with if
the circuit might be live. As long as you have an insulated screwdriver
(or tape up your regular one), it seems like you'd have to work at it a
bit to touch an exposed conductor. I know that in super perfect happy
world, this doesn't matter, because all of the electricity is off when
you are doing wiring. In the real world...

Thanks!

Matt Roberds

 

[Andrew Gabriel]

Makes sense. I knew they had a maximum wire gauge but I didn't know it
was OK to just add them up. (This is a lot easier to figure out when
your wires are known by their cross section in real units, rather than
by gauge.)

Actually, I should say that in more recent years, the cross-sectional
rating has given way to a current rating. I'm not too sure what the
mapping is, but by observation, a block rated at, say, 15A, will very
comfortably take at least a couple of 15A conductors in each side.
(This is in stark contrast to DIN-rail mounting terminals, where a
15A terminal will take exactly 1 15A conductor, and it's impossible
to fit anything else in.)

Ah, *that's* what that piece of metal is for. I had seen it before,
often on terminals that are used to connect small-ish (<0.8 mm^2 or so)
loose wires to a circuit board, but didn't quite understand why it was
there.

It's also important for tiny solid core conductors used for signalling,
telephony, etc, where a clamp screw alone can easily break the conductor.

The "chocolate block" connectors also seem less scary to work with if
the circuit might be live. As long as you have an insulated screwdriver

Yes, I do that reasonably often. The main risk is the existing live
conductors popping out when you undo the screw, as you don't necessarily
know what tension is on any of the existing cables until you do that.
I wouldn't do it with anything at mains level fused at more than a few
amps though, e.g. I might do it with a domestic lighting circuit here
which is normally protected at 6A, but not with a socket outlet circuit
which is protected at 32A - the bang would be too big!

 

[.]

Actually, I should say that in more recent years, the cross-sectional
rating has given way to a current rating. I'm not too sure what the
mapping is, but by observation, a block rated at, say, 15A, will very
comfortably take at least a couple of 15A conductors in each side.
(This is in stark contrast to DIN-rail mounting terminals, where a
15A terminal will take exactly 1 15A conductor, and it's impossible
to fit anything else in.)


It's also important for tiny solid core conductors used for signalling,
telephony, etc, where a clamp screw alone can easily break the conductor.


Yes, I do that reasonably often. The main risk is the existing live
conductors popping out when you undo the screw, as you don't necessarily
know what tension is on any of the existing cables until you do that.
I wouldn't do it with anything at mains level fused at more than a few
amps though, e.g. I might do it with a domestic lighting circuit here
which is normally protected at 6A, but not with a socket outlet circuit
which is protected at 32A - the bang would be too big!

Andrew, can you provide a link to pictures of the various "chocolate block"
designs? I'll be attending a meeting of the NEC Code Panels in a few weeks
as the new NEC code cycle gets underway. I'd like to see what the thinking
is regarding blocks and wire nuts and to see if there's any interest in
harmonizing such wiring practices and products.

Thanks,

Terry McGowan

 

[Andrew Gabriel]

Andrew, can you provide a link to pictures of the various "chocolate block"
designs? I'll be attending a meeting of the NEC Code Panels in a few weeks
as the new NEC code cycle gets underway. I'd like to see what the thinking
is regarding blocks and wire nuts and to see if there's any interest in
harmonizing such wiring practices and products.

It's quite hard to find a good picture, but I came across a datasheet:
http://www.farnell.com/datasheets/13864.pdf

Mostly, they don't have the leaf protection spring as it's not needed
for the cable used for normal wiring, but this datasheet shows how
the optional leaf protection spring works.

There are lots of different types of insulator materials, some quite
soft and flexible, others much harder with higher temperature ratings,
but they're all designed to be able to be sliced through with a knife,
to cut the 12-way blocks down into the number of connectors required.

Also, you get different height stand-offs on the bottom, depending if
any degree of stand-off is required to meet creepage distances, e.g.
if fixed to metalwork or inside an insulated environment.

There's quite an ecosystem of supportive products too, such as pop-
together enclosures with a couple of molded in thin rods to engage
in the terminal strip mounting holes, and you also often find
appliance plastic casings have these too, to terminate the mains
flex to the internal wiring.

Some more pictures:
http://image.made-in-china.com/2f0j00NPTtGbQlRagF/U-H-F-Type-Terminal-Blocks-Connector-Strips-.jpg
http://javedplastic.com/images/products/panel-box-items/terminal-blocks.jpg
http://www.boltproducts.com/heyco/wire-standard-terminal-blocks.html
http://www.sciencephoto.com/media/343424/enlarge

ceramic ones:
http://www.hawco.co.uk/Ceramic-Terminal-Block_1242_p.aspx
https://www.homespare.com/content/thumbnails/i_200058_1.jpg

 

[Nono]

Terry,

Connector blocks like these are sold in the US through distributors
such as Newark, Allied, etc. They may even be sold by Grainger.
However, ours are white, so I don't understand the "chocolate block"
nickname. Is it because they can be cut or broken apart to make
smaller "pieces" ?

A recent visit to Home Depot was instructive, Vic. There are several types
of "block" connectors available, as you say, including a nifty weather-tight
version that can be used to splice Type UF wire and then buried in the
ground.

Terry McGowan

 

[Andrew Gabriel]

Terry,

Connector blocks like these are sold in the US through distributors
such as Newark, Allied, etc. They may even be sold by Grainger.
However, ours are white, so I don't understand the "chocolate block"
nickname. Is it because they can be cut or broken apart to make
smaller "pieces" ?

Yes, broken up like a bar of chocolate (although you need a knife).

 

[Nono]

Yes, broken up like a bar of chocolate (although you need a knife).

It's white chocolate, Vic. I'm sure there's a requirement in the National
Electrical Code about that -- written by a candy lover.

Terry