Oi! Nigel Calder! NO...

[Dominic_Byers]

Dear All,

I have a reply from Nigel Calder himself: Don't foget that you are all welcome to send any comments or views directly to Yachting Monthly (address in front of mag) where they will be considered for publication and will receive a reply and answer.

Nigel writes:

My comments about the fire risk created by adding a stainless steel washer
between a high current cable lug and the fuse or busbar to which it is
bolted seem to have caused a bit of heat! These comments were based on
practical experience, not electrical calculations (I am not an electrical
engineer). When I saw the various comments about 'scaremongering', I
wondered if what I have seen may have had other causes, so I consulted an
electrical engineer who runs a company that manufactures high current
industrial control systems, and who routinely bench tests the kinds of
situation I described. Here are some of his comments:
1. In any junction, you have electrical and mechanical issues. If you put a
washer in the circuit, you have twice as many potential poor connection
spots, so this is not just an electrical conductivity issue.
2. Washers are not flat, nor uniform in thickness. The degree of mechanical
connection will depend on these factors, and also on the degree of crush
applied to the washer (many of which are distinctly cupped). Mechanical
imperfections are likely to introduce significant resistance to the connection.
3. The thermal conductivity of 300 series stainless steel (which is what is
mostly used for washers) at 20 degrees centigrade is in the mid 70's times
ten to the power of minus 6, as opposed to copper which is 1.7 times 10 to
the minus 6 - i.e. stainless has approximately 40 times the resistivity of
copper. As the temperature rises, he thinks the relative resistivity of
stainless rises.
4. If we assume a one sixteenth inch thick, one inch diameter stainless
steel washer, with a half inch hole, and full contact between the washer
and the cable lug and terminal block (all of which assumptions likely
overstate the contact area by a considerable amount), with a perfect
mechanical connection, carrying a 300 amp current, the heat generated is
0.27 watts. Depending on the heat dissipation (often poor, because the
cable is insulated, in addition to which if the connection is to a fuse in
a phenolic block there will be little dissipation on this side of the
connection - this is a typical inverter connection), he likened this low
level of heat to a drip that eventually fills a bathtub. In practice, with
smaller contact areas and a less than perfect mechanical connection, the
heat input will be significantly higher.
5. If the connection is to a fuse or other terminal mounted in plastic, as
the temperature rises the plastic softens, the connecting bolt loosens, the
mechanical connection degrades, the resistance goes up, and the temperature
rises faster. Zytel, apparently a common fuse holder plastic, softens at
130 degrees centigrade.

The bottom line is, he strongly recommends keeping stainless steel washers
out of high current circuits! Coming back to my starting point, the first
time I realized this was a problem was when I found my high current busbar
too hot to touch - the problem was a stainless steel washer under the
inverter connection (which was tightly bolted down to a busbar, as opposed
to being in a plastic holder). Subsequently, I have seen two small fires,
both on bolted connections to fuse blocks rather than to busbars (I now
suspect that the fires were probably generated by the arcing that likely
occurred when the heat softened the plastic and loosened the mechanical
connection...). I have heard of similar situations from others in the
industry. It would be nice to do some real world testing, but I don't have
access to the necessary equipment at the moment. Anybody want to give it a try?

So there you have it. Is this scaremongering? I don't think so!! I'd get
those washers out of there, particularly since they serve no purpose...

Nigel

Ends

 

[ean_p]

Mmmmmm would have to disagree with NC's friend on the pointthat the use of a washer simply ' doubles the poor connection spots'. Have just left an industry after 12 years wherein high current (up to 420+amp) connections were common place and both here in the UK where units were maintained and repaired and in Germany where the units were manufactured it was the norm to always always use a washer on both stud and screw connections. But in every case these washers were machined, not stamped so as to ensure reasonably flat surface and they were inveriably made of brass or copper. Off the top of my head it would be unusual to find a nut that had an area anywhere like as large as a similar sized cable shoe and hence another reason to fit washers which would......anyway on to another subject....in a steel boat would you earth the neg side of your batteries to the engine block etc while still running a two cable system , or would you leave both sides of the battery isolated from the rest of the boat .......don't worry will be posting this on the board as well.......any pro's and/or cons for either method????

 

[pvb]

Now I\'m really mystified!

Many thanks, Dominic, for taking the trouble to get Nigel Calder’s comments. But I’m completely mystified by his remarks, which are largely incorrect.

After his scare-mongering about stainless steel washers, we now have scare-mongering about the plastic in fuseholders softening and causing the bolted connection to become loose and cause a fire, and Nigel tells us that “Zytel, apparently a common fuse holder plastic, softens at 130 degrees centigrade”. This is totally untrue!

Zytel glass-filled PA66 nylon softens at around 250 C (depending on the percentage of glass reinforcement), roughly twice the temperature Nigel claims! (Don’t take my word for it, you can quickly check it on the internet, details below). The excellent heat resistance of Zytel is one of the reasons why it’s specified for good quality fuse holders (such as those made by Blue Sea). Incidentally, as the plastic insulation on high quality marine cable is only rated at 105 C, won’t the cable melt and cause a problem long before a Zytel fuse holder softens?

It’s also generally untrue for Nigel to say that if the plastic softens, the mechanical connection will come loose. Every half-decent high current fuse holder or busbar is designed so that the plastic base is merely a mounting and is not vital to the mechanical or electrical integrity. I was concerned enough about his allegations to spend several hours today checking the Blue Sea and Littelfuse fittings on my boat - wasted time, because sure enough the plastic plays no part at all in the strength of the bolted connections. Maybe Nigel should name the brands which he claims will melt and come loose, so that we can avoid buying them.

Finally, back to the stainless steel washer issue. Any engineer will confirm that a heating effect of a fraction of a watt caused by a stainless steel washer will not overheat a busbar. Not ever. And Nigel’s friend’s analogy of the “drip that eventually fills a bathtub” is inappropriate - water doesn’t leak out of a bath, but heat constantly escapes from a busbar by both radiation and convection. If you’re still undecided, just consider the engine instruments on your boat. They have little light bulbs inside for night-time use. The bulbs are typically rated at around 1 watt, and create about half a watt of heat. Do your instruments overheat at night? Does their plastic casing melt? Do they catch fire? Exactly.



PS
Zytel is made by DuPont whose website (http://www.dupont.com/enggpolymers/english/products/zytel.html) gives full details. You need to look for the “Vicat softening temperature”. The Vicat (ISO306) test is the worldwide standard for measuring the heat-related softening of thermoplastics.