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PDP-8/e earth leakage problem

Mal

Experienced Member
Joined
Apr 3, 2008
Messages
91
I recently moved house and found that my PDP-8/e triggered the RCD (Residual Current Device) in the switchboard of my “new” (15 year old) home.

The problem was quickly identified to be the Sprague 0.1uF “line-to-earth” capacitors in the 8/e’s H724 Power Supply.

In summary: the line-to-earth capacitors cause an earth leakage current that exceeds the trip threshold for (now compulsory) RCDs in Australian homes.

The fix was to simply remove the Sprague capacitors. However, this leaves 11 mains-voltage spade connectors floating inside the H724 power supply. Not ideal.

So I’ve drawn up a simple PCB that replaces the Sprague capacitors with a suitable array of terminals for the spade connectors. It also optionally provides for a small array of 4.7nF Class Y line-to-earth safety capacitors that may offer some RFI suppression benefits (albeit much reduced from the larger original Sprague capacitors).

A more detailed commentary, and PCB artwork, are available here -> http://avitech.com.au/?page_id=3585

This small modification reduced the earth leakage current from 13-21mA, down to 0.7mA. Earth leakage problem solved, though admittedly I really don’t yet understand whether I have introduced a significant RFI problem in doing so (by reducing the line-to-earth capacitors from 4x100nF to 4x4.7nF).

Malcolm.
 

g4ugm

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Joined
Feb 22, 2011
Messages
2,793
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NorthWest England (East Pondia)
I recently moved house and found that my PDP-8/e triggered the RCD (Residual Current Device) in the switchboard of my “new” (15 year old) home.

The problem was quickly identified to be the Sprague 0.1uF “line-to-earth” capacitors in the 8/e’s H724 Power Supply.

In summary: the line-to-earth capacitors cause an earth leakage current that exceeds the trip threshold for (now compulsory) RCDs in Australian homes.

The fix was to simply remove the Sprague capacitors. However, this leaves 11 mains-voltage spade connectors floating inside the H724 power supply. Not ideal.

So I’ve drawn up a simple PCB that replaces the Sprague capacitors with a suitable array of terminals for the spade connectors. It also optionally provides for a small array of 4.7nF Class Y line-to-earth safety capacitors that may offer some RFI suppression benefits (albeit much reduced from the larger original Sprague capacitors).

A more detailed commentary, and PCB artwork, are available here -> http://avitech.com.au/?page_id=3585

This small modification reduced the earth leakage current from 13-21mA, down to 0.7mA. Earth leakage problem solved, though admittedly I really don’t yet understand whether I have introduced a significant RFI problem in doing so (by reducing the line-to-earth capacitors from 4x100nF to 4x4.7nF).

Malcolm.

Probably. Modern RF filters don't conduct so much to ground. Filters are designed to be matched to the impedance of the load. By reducing the capacitors you have changed the match. I would look to replacing the filter with a modern design e.g. https://uk.rs-online.com/web/p/power-line-filters/2192791/
 

bobaboba

Experienced Member
Joined
Sep 7, 2013
Messages
393
Location
Scotland
I have a separate mains feed to my shack bypassing the rcd especially for my vintage gear, so the old mains filter arrangements don’t trouble me.
 

g4ugm

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Feb 22, 2011
Messages
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I have a separate mains feed to my shack bypassing the rcd especially for my vintage gear, so the old mains filter arrangements don’t trouble me.

To install this as new in the UK would new now be illegal and invalidate any insurance. Since the beginning of the year all circuits require an RCD or a signed off risk assessment saying they are not needed. Given the nature of a Ham Shack I can't see how you could risk assess away an RCD.
 

daver2

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Jun 19, 2012
Messages
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Location
UK - Worcester
As I read the new version, the clause that could apply to vintage computer equipment is "(b) for a specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment.".

It may be that a separate cable without an RCD to a bespoke socket for connection of (say) a DEC 11/45 would still be permitted. I am not planning to do this of course, but (if it was absolutely necessary) then there is a potential clause you may be able to use...

Dave
 

g4ugm

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Joined
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Messages
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Location
NorthWest England (East Pondia)
As I read the new version, the clause that could apply to vintage computer equipment is "(b) for a specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment.".

It may be that a separate cable without an RCD to a bespoke socket for connection of (say) a DEC 11/45 would still be permitted. I am not planning to do this of course, but (if it was absolutely necessary) then there is a potential clause you may be able to use...

Dave

The IET guidance states that

https://electrical.theiet.org/wirin...rements-for-rcd-protection-of-socket-outlets/

"It is stressed that the exception may only be applied for a socket-outlet that has been provided for the connection of a particular item of equipment. The socket-outlet must be labelled or otherwise suitably identified so that users will be clearly informed that the socket-outlet is intended only for plugging in that particular item of equipment and for no other purpose. Unless the electrical installation designer is convinced that the socket-outlet cannot reasonably be expected to be used for other purposes, RCD protection for that socket-outlet should not be omitted.

In the vast majority of cases, it should not be necessary to omit RCD protection for a socket-outlet. For compliance with Regulation 531.2.4, a socket-outlet should be connected to an RCD that serves a sufficiently small number of other socket-outlets or items of equipment, so that any protective conductor current that may be expected in normal service will be unlikely to cause unwanted tripping of the RCD."


I would say in a radio shack or computer museum with multiple bits of gear that requirement is hard to satisfy. I would also suggest that having earth leakage protection where there is lots of vintage kit around is a good thing. My shack sockets are on a separate RCBO and if it trips it only takes out the shack. It did trip recently because a capacitor in a Creed TTY motor filter went leaky. I am going to have to replace it....
 

TJ_Mossman

Experienced Member
Joined
Dec 1, 2013
Messages
369
Location
Bristol, England
The IET guidance states that

https://electrical.theiet.org/wirin...rements-for-rcd-protection-of-socket-outlets/

"It is stressed that the exception may only be applied for a socket-outlet that has been provided for the connection of a particular item of equipment. The socket-outlet must be labelled or otherwise suitably identified so that users will be clearly informed that the socket-outlet is intended only for plugging in that particular item of equipment and for no other purpose. Unless the electrical installation designer is convinced that the socket-outlet cannot reasonably be expected to be used for other purposes, RCD protection for that socket-outlet should not be omitted.

In the vast majority of cases, it should not be necessary to omit RCD protection for a socket-outlet. For compliance with Regulation 531.2.4, a socket-outlet should be connected to an RCD that serves a sufficiently small number of other socket-outlets or items of equipment, so that any protective conductor current that may be expected in normal service will be unlikely to cause unwanted tripping of the RCD."


I would say in a radio shack or computer museum with multiple bits of gear that requirement is hard to satisfy. I would also suggest that having earth leakage protection where there is lots of vintage kit around is a good thing. My shack sockets are on a separate RCBO and if it trips it only takes out the shack. It did trip recently because a capacitor in a Creed TTY motor filter went leaky. I am going to have to replace it....

I'm not sure of the specific rules relating to this, but in my last workplace in 2017 we had a similar problem and opted to use two non-standard 13A sockets, one with a T-shaped earth pin, and another with a round earth pin. The electrician was satisfied as it was impossible to plug a device into the wrong socket this way (unless someone were to change the plug), thus negating the risk of someone ignoring the specific labeling on the plug or socket.
 

AK6DN

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Joined
Aug 23, 2010
Messages
1,066
Location
Silicon Valley USA
I had a problem with my PDP-8m tripping the GFI outlet (US version of RCD I believe) it was plugged into. All my available garage circuits were GFI, no other option. So I inserted a 1000W isolation transformer inline with the PDP-8m power, until I was able to debug the power supply months later and figure out the issue (leaky filter caps line/ground in the input AC filter box).
 

g4ugm

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Feb 22, 2011
Messages
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NorthWest England (East Pondia)
I'm not sure of the specific rules relating to this, but in my last workplace in 2017 we had a similar problem and opted to use two non-standard 13A sockets, one with a T-shaped earth pin, and another with a round earth pin. The electrician was satisfied as it was impossible to plug a device into the wrong socket this way (unless someone were to change the plug), thus negating the risk of someone ignoring the specific labeling on the plug or socket.

Be a little when applying rules from workplace to home. There are many subtle differences between the rules for residential, and non-residential properties. So in the UK it would be perfectly legal for you to totally re-wire a scout hut, so long as it was free standing, so not residential, no part p, but if you touch your consumer unit in your home, as thats residential and the protection in it (RCD/RCBO/Breakers) you need a Part P notification....
 

bobaboba

Experienced Member
Joined
Sep 7, 2013
Messages
393
Location
Scotland
You need to consider ‘legal’ in this context as my wiring predates any rcd requirements, wouldn’t require retrofitting, and, as I live in Scotland, part P isn’t applicable and wiring regs are recognised good practice rather than mandatory (lapsed now but I used to be NICEIC with my own business). In my house the fridge is on its own non-rcd circuit, the freezer in the garage is non-rcd and my shack has both an rcd and non-rcd feed (everything’s labelled for someone else’s convenience).

Doesn’t prevent me attaching myself to a non-rcd live feed inadvertently of course as I’ve done a few times over the years, but all quite legal.
 
Last edited:

Hugo Holden

Veteran Member
Joined
Dec 23, 2015
Messages
2,999
Location
Australia
There really is no problem if you have a single appliance with some vintage design characteristic which trips a modern RCD. Simply this:
Make sure every GPO in the house or shack is protected by an RCD as now required in many countries, and preferably have RCD on lighting circuits too.This is the safe thing to do.
However, you can legally run any SINGLE appliance from an approved isolating transformer (never plug more than one appliance onto the said transformer). As it turns out, the isolation afforded by the transformer defeats the RCD function and the device/appliance then won't trip your RCD.
 
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