Re: Auto Starting Mac

[Neil Houghton]

Hi Kaye,

 

Hmmm – at this point I would be wondering about some hardware/circuit 
intermittent or strange fault/problem.

 

In a previous life I worked with ROVs  
https://en.wikipedia.org/wiki/Remotely_operated_underwater_vehicle  offshore – 
and I well remember a fault which defied my trouble shooting for a while – not 
a popular outcome if an offshore oil rig is waiting for you to put a working 
machine in the water!

 

This particular ROV is powered by 1000v 3-phase down the umbilical which then 
drives a hydraulic system of over 100HP for propulsion etc. Given the 1000v 
3-phase subsea, at depth, and the crew all working on a steel deck we had to 
have good RCD protection – like the RCDs in your house (providing the wiring is 
relatively modern) which shuts down the power if an earth leakage fault is 
detected. For the electronic control systems the 100v got transformed down to 
115v and had its own RCD protection.

 

My problems was that, on deck, the system checked out just fine – all pre-dive 
checks OK, everything working, no problems. But as soon as the ROV was lowered 
into the water, the 115v RCD tripped out. Back on deck, open up everything – no 
water ingress, reset RCD – no problem, re-run all pre-dive checks – all OK.

 

Re-launch ROV – hits the water – RCD trips!

 

This went on for a while, no problem could be found until one of the techs 
suddenly exclaimed “I just got a zap off the pod” – he was not too impressed 
when, instead of sympathy, my reaction was “you beauty!” – I now had something 
real to investigate.

 

I will spare you the all details but, basically, the control electronics 
chassis slides in and out of an aluminium pressure vessel (the afore-mentioned 
“pod”) and all electrical connections to the chassis are via submersible 
connectors in the chassis end plate, which is O-ring sealed to the pressure 
vessel. As protection from shocks/vibration, the pressure vessel is mounted in 
the ROV via rubber pads. The pressure vessel is hard-anodised inside and out 
and the anodized film is electrically non-conductive.

 

What all this means is that neither the ROV chassis nor the electronics 
chassis, which are both earthed, have any electrical connection to the actual 
pressure vessel which, having no direct electrical connection to anything else, 
was not earthed. The fault turned out to be that somehow, during the repeated 
insertion/removal of the electronic chassis from the pressure vessel, a small 
amount of insulation had been scraped off one of the 115v supply wires and, 
coincidentally, the position of the insulation break just happened to line up 
with a scratch in the anodic coating on the inside of the pressure vessel – 
making an electrical connection from the aluminium body of the pressure vessel 
to the 115v supply.

 

Since the outside of the pressure vessel is also hard anodised and 
non-conductive this did not result in any detectible fault or malfunction 
whilst on deck. However, due to general wear and tear, there are always small 
chips/breaks in the anodic coating (typically corners/edges get knocked) so 
when the ROV hit the water, the seawater provided the earth leakage path from 
the (now) 115v connected pressure vessel body and the earthed ROV chassis – 
tripping the RCD.

 

When the hapless tech happened to brush past one of the external chips in the 
anodic coating he got a slight “zap” hence his exclamation pointing me in the 
right direction!!

 

This elusive problem & its (final) resolution was oft remembered in the 
well-lubricated reminiscences when the old crew caught up!

 

 

Not that this provides any guidance as to YOUR elusive problem – but maybe some 
checking around the on/off button on the iMac might be worthwhile – stray bit 
of wire, sticky button, condensation etc etc….

 

 

Cheers

 

 

Neil

 

 

From:  on behalf of kaye and 
geoff 
Reply-To: 
Date: Tuesday, 26 May 2020 at 18:37
To: , “WAMUG_Mailing_List” 
Subject: Re: Auto Starting Mac

 

Well,

 

We're back in X-factor country. I checked out the iMac today and "Start up 
automatically after a power failure" was off!

 

I was sure it would have been the answer. I don't set up my machines with it 
on, but it COULD have happened, and if it had been set it would have been the 
perfect answer.

 

More tests needed.

 

Cheers, Kaye



Kaye and Geoff

k...@kgweb.org.au

 

 

 

 

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Re: Auto Starting Mac

[kaye and geoff]

Well,

We're back in X-factor country. I checked out the iMac today and "Start up 
automatically after a power failure" was off!

I was sure it would have been the answer. I don't set up my machines with it 
on, but it COULD have happened, and if it had been set it would have been the 
perfect answer.

More tests needed.

Cheers, Kaye

Kaye and Geoff
k...@kgweb.org.au





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