Resistor in series with mains earth? (HP equipment)

[cmjones01]

I'm resurrecting an HP 334A distortion analyzer bought cheaply in a disposal auction. It may be old, but it has some useful properties for what I want to use it for, and it's pretty. This particular example has components dated 1968 inside it.

I've just been replacing the annoying HP round-pin mains inlet with a modern IEC plug, and noticed that the earth connection has a 1 ohm resistor between the mains inlet and the chassis! I'll leave it as-is for now, though it would clearly fail a PAT test, but why on earth (ha!) would they do this? It doesn't seem like a high enough value to usefully break an earth loop.

Any insight would be much appreciated.
Chris

[broadgage]

I have read in an old electrical engineering textbook of the practice of inserting a small resistance in series with the mains earth.

The intention was to limit the fault current and thereby reduce the damage caused.
With no series resistance and a very low supply impedance, a fault current of thousands of amps could flow and cause significant destruction in the short time before the fuse operated.

With a one ohm resistance the fault current would be limited to about 250 amps, enough to promptly open a 15 amp fuse.

[Alistair D]

Looking at this table https://www.pat-testing-training.net...cteristics.php a 13A plug fuse may never blow at 20A. That would leave the 1 ohm resistor dissipating 400 watts. Under those conditions, unless the resistor is very heavy duty, I suspect the resistor would fail very quickly leaving any exposed metalwork at mains voltage.

In my opinion It should be removed before ever powering the instrument up.

Al

Edit: I realise that the instrument will probably have it's own much lower rated fuse. With the different mains lead wiring colours it is possible that L and N could be reversed putting the instrument fuse in the Neutral wire. In that case the only fuse in the Live lead will be the plug fuse.

[Herald1360]

Why not simply fit a 3A fuse in the mains plug?

(Presumably this piece of kit is only going to be used by someone with a bit of nous).

[Skywave]

I've met low resistance RF chokes in earth wiring to a.c. mains input, but the fitting of that resistor where it is seems like a daft idea to me. I'd take it out and arrange the earthing in the now conventional manner.

Al.

[turretslug]

I'd go along with the "remove the resistor and connect earth directly to chassis" sentiment- as the instrument now has an IEC inlet anyway, it won't cause any further offence to fit a currently-compliant mains fuseholder with a suitably low-current fuse to guard against the IEC-lead-that-someone's-fitted-with-a-13A-fuse syndrome.Whilst I have some respect for the originality philosophy, I won't hesitate to fit a primary-side fuseholder if originally absent.

[John10b]

That’s very interesting Alistair D thank you for the C&G information.
Cheers
John

[cmjones01]

Thank you everyone for the thoughts. I can see no reason not to connect the mains earth directly to the chassis, especially since the inputs and outputs all appear to be isolated from the chassis anyway so this instrument won't be contributing to any earth loops.

There is already a primary side fuse, rated at less than 100mA (!).

Just to add interest, in my home workshop, nothing is earthed anyway. It's in a room where all the sockets are 2-pin but they accept standard European earthed plugs (French-style, as used here in Poland, and also German Schuko plugs). This is quite common in central Europe. So in 'dry' areas, away from the kitchen and bathroom, nothing is earthed and you have no idea which way round live and neutral are. Testing for earth leakage is simple and intuitive. Hold on to the radiator and touch the case of a nearby piece of equipment. If you remain standing, the earth leakage current is acceptable!

Chris

[ionburn]

I remember coming across this years ago in a computer monitor. I cannot remember how I got around as regards testing but do remember having to think at the time.

[factory]

All the information is in the service manual's backdating section.

Before 1968 there was a choke fitted, some 1968 models have the 1 ohm resistor, later than that and the resistor is gone.

Not sure it would have been a problem anyway with the original PH-163 socket, as they have a metal housing & the earth pin is usually directly connected to this.
I imagine your replacement IEC socket has a plastic housing (later models have these too), hence why the resistor is no longer fitted.

Note: These distortion analyzers have a separate inner chassis which is isolated from the outside casing.

David

P.S. I prefer to keep the original mains sockets in my older HP test gear.

[Restoration73]

I was restoring an old Advance SG63 signal generator, and replaced the mains lead.
It failed the portable appliance test as each of the 3 mains conductors were connected
via rf chokes mounted on a tagstrip near the cable entry. I decided to solder a link
across the choke in the earth conductor.

[factory]

The mains fuse for the HP 333/334A is 62mA for 220-240V, 150mA for 100-120V.
Both my 331A & 333A have terminals on the back for external battery power too, the 331/332 use lower fuse values, but the manual has an error as one diagram shows the higher values.

David

[cmjones01]

Originally Posted by factory

Not sure it would have been a problem anyway with the original PH-163 socket, as they have a metal housing & the earth pin is usually directly connected to this.

The earth pin of the inlet is definitely not connected to its metal shell, but if the cable-mounted socket was metal and earthed, that would neatly solve the problem.

I imagine your replacement IEC socket has a plastic housing (later models have these too), hence why the resistor is no longer fitted.

Yes, it's plastic, so I'll modify the wiring to the post-1968 form.

P.S. I prefer to keep the original mains sockets in my older HP test gear.

I was tempted to do this too, but finding matching mains leads in good condition seemed to be tricky and expensive. I only have two items of gear which used this PH-163 connector and they both have to be practical and safe to use in day-to-day work, so I chose to modify them. Neither of them is rare, and their value to me is hugely increased by making them useful and reliable.

Chris

[factory]

Originally Posted by cmjones01

The earth pin of the inlet is definitely not connected to its metal shell, but if the cable-mounted socket was metal and earthed, that would neatly solve the problem.

That is why I said usually and not always.

HP Part no. #1250-0148 has the earth/ground pin attached/connected to the metal housing.


HP Part No. #1250-1009 as used in the older 33xA analyzers isn't connected as you've found, this makes perfect sense as they fitted the choke or resistor.

I was tempted to do this too, but finding matching mains leads in good condition seemed to be tricky and expensive. I only have two items of gear which used this PH-163 connector and they both have to be practical and safe to use in day-to-day work, so I chose to modify them. Neither of them is rare, and their value to me is hugely increased by making them useful and reliable.

Chris

None of mine are used day-to-day for work, they are part of my collection, I have many items that use the PH-163 connector and other obsolete connectors, hence I bought enough new leads (before they were discontinued by Volex) for my own use.

Note: old IEC leads and connectors can also go bad, don't assume that because they are the current standard that they are safe, check & inspect regularly, as you would do with anything electrical.

David

[factory]

Just had a quick look inside the older 331A (approx 1966) on the repair pile here, it's got an inductor connected to ground, which has four turns of thick wire, the resistance is too low for a multimeter to measure (needs a low-ohm meter).
I won't be removing it, but if it had a 1 ohm resistor (as the OP's did) then it would be going.

David

[Stockden]

I remember coming across a similar arrangement in a Solartron AS1412 psu. A glance at the circuit diagram suggests that in this case it has a bypass link.

I must check which way that link is set on my unit!

[Radio Wrangler]

The 'NEMA' connectors on the mains leads that plug into those old round pin receptacles seem to be housed in hard rubber. I've never seen one with a metal outer, grounded or not.

The IEC cable end has problems. The earth pin is longest and takes an unfair proportion of any stress, this spreads the contact tines and reduces their pressure and springiness.

After an electric shock incident (filter capacitance leakage current of several instruments combined) HP had a test fest of all the IEC cables in the plant. 10% were unacceptable, failing a 25A bonding test. All bad ones were replaced with new. The bad ones were destroyed. A year later, we tested again.... 10% the next year.... 10% I thought I'd stumbled on a new physical constant!

Wandering leads frequently moved around on benches and moved from equipment to equipment... 10% Static cables used in racks of non-moving equipment 10%.


THis info was obtained from a population of many thousands of cables spread across laboratory, office and production environments. Failures did not correlate with the environment.

In the end we date coded all cables and dumped them after 2 years.

David

[factory]

The connector that goes in the older HP test gear is PH-163 not NEMA.

NEMA is the connector that goes in US wall outlets and replaced with a UK plug over here.

Both Alpha & Volex did make PH-163 cordsets in PVC.

More information here for Volex PH-163 cords;
https://www.eevblog.com/forum/testge...90/#msg2389290
And Alpha PH-163 cords:
https://www.eevblog.com/forum/testge...47/#msg2391747

At work both IEC leads (plenty) & sockets (a few) did fail the earth bond test, they solved that problem by outsourcing testing, now they never fail as they never turn up with the earth bond lead.

David

[cmjones01]

David, that's very interesting, if only to demonstrate that there is a point in regular PAT testing!

Chris

[turretslug]

Mind you, both NEMA and IEC are organisations whose abbreviations get used as colloquialisms for specific connectors- most folk in the business get accustomed to what is being referred to, though strictly speaking we ought to be more specific as in IEC C13/14 connector etc.