PME and RF Galvanic Isolation

[russell_w_b]

Originally Posted by David Simpson

Here endeth today's 2nd lesson. Prahaps,to please the moderator, if anyone wants more detailed advice on PME & associated circuitry - send me a PM, & I'll try & help. I'm nae wiser than anyone else, just fortunate in having been trained in Electronics(Radio Comms & Test Equipment), and HV & LV Electricity Supplies.

Regards, David

David,

Many thanks for answering my question! I understand that cattle are indeed sensitive to potential gradient. Better get back on topic now, I suppose...

[Skywave]

Originally Posted by David Simpson

. . the provision of an "Earth" in a domestic electrical circuit is not necessaraly to prevent a person getting an electrical shock - it is to ensure that the overload current protection devices operate as quickly as possible. The only dedicated device to protect a person is an RCD. . . .
Regards, David

You obviously know a lot more than I do on this subject, but - with regard to the above - "earthing" of the electrical supply to domestic premesis was done long, long time before RCDs were invented.
Picking up on that - and John Robson's Post - surely the whole idea of running an earth wire into a domestic installation is simply a safety feature - almost along the lines that John R stated. One side of the incoming mains is at earth at the substation. Should the line / "live" wire in a piece of equipment contact its earthed metal casing, it will simply blow a fuse / trip the RCD - and save the user from electrocution - who will be in physical contact (or nearly so) with "earth". If the incoming mains was balanced to earth and there were not any earth connections anywhere on the load side of the sub-station transformer, an earth lead would not be required - which, in essence, is the result when an isolating transformer is used in a domestic installation to power various item(s) that the consumer chooses to so connect.
In which case, the question arises: why does the secondary side on the transformer at the sub-station have one side of its winding connected to earth anyway? Why not simply run 4-wire balanced 415v a.c. line-line 3-phase to consumer installations, producing 2-wire 230v a.c. phase - neutral balanced feeds?

I'm sure you'll educate me on these points - thank you.

Al / Skywave.

[russell_w_b]

Originally Posted by Skywave

In which case, the question arises: why does the secondary side on the transformer at the sub-station have one side of its winding connected to earth anyway? Why not simply run 4-wire balanced 415v a.c. line-line 3-phase to consumer installations, producing 2-wire 230v a.c. phase - neutral balanced feeds?

I'm sure you'll educate me on these points - thank you.

Al / Skywave.

You mean have the consumer connected to two phases of a delta secondary winding rather than a star-winding with earthed neutral?

Well, if the incoming mains was balanced (ie: no particular phase was neutral), then a faulty appliance with a wire touching the case would go un-noticed by whoever was touching it - until they touched (or the faulty metallic case of the appliance touched) another piece of faulty equipment with a wire from the other phase touching its case! This is why it's not good practise to connect a multiway extension block to one of these little generators with floating supplies. Faulty appliances go un-noticed until someone picks up two faulty appliances on the extension block; one with a live-to-earth fault and the other with neutral-to-earth fault - Arghhh!

Similarly, if one wire of a balanced system supply made inadvertent and unbeknownst contact with earth, then the system becomes unbalanced and provides a path from the phase in the faulty appliance, through the unfortunate person holding it, through earth, and back to the other phase through the inadvertent and unbeknownst supply earth fault.

If one phase is solidly referenced to earth, the above won't happen (assuming they're wired correctly, of course!).

[Skywave]

Thanks Russell.

As you've probably guessed - I hadn't thought of those scenarios that you so lucidly explain. Also, I now stand educated on why it is not a good idea to run more than one item from a mains-powered isolation transformer - unless all items are in metal cases and the metal cases are substantially bonded together (this becomes clear in a quick drawing).

Incidentally, I was thinking of star-connected secondaries at the sub-station without a connection from the star point to earth - viz. a truly floating neutral - but your reasoning covers that situation also.

Al / Skywave.

[David Simpson]

Guys, Have consulted an ex BT Telecomms Station Manager. Refering to the days before satelite & microwave parabolic antennas, all HF & VHF stations had 3 "Earths" :- 1) Electricity Supplier's Earth - for all building's ring mains, lighting,space heating,etc - and complying with IEE & PME or CEW Regulations earthing & bonding wise. The mains supplied were usually 3ph 4wire star 415V .
2) The Station's "Dirty" Earth :- All rotating machinery, pumps, compressors, s/by gennies etc,installed in an annex or separate building, had it's own dedicated earth.
3) Each Aerial had it's own Earth Matt(Woven Copper Mesh),buried adjacent or under the Aerial pole or mast. All these Matts could be bonded together underground - depending on local earth conductivity conditions.
NONE of these 3 "Earths" were connected to another! This configuration proved for many years to be the best & safest for transmisson & reception purposes.
Refering to choice of Star or Delta mains supplies - "Delta" is dedicated to 3phase 11 & 33KV(or other HV voltages) overhead networks or u/g supplies to sub-station's primary windings. When required for rotating machinery star/delta switchgear is used. After the supply has been dropped to LV voltages from the rural transformer or urban sub-station's secondary windings, the supplies are distributed in "Star" configuration. Some inbalance will always occur, but with experience & applied maths - distribution engineers always seem to keep this to a minimum, when designing or modifying LV distribution networks. Mind you, they are usually pretty hot on ensuring that commercial premise's have the correct Power Factor correction devices installed.
Regards, David

[russell_w_b]

Originally Posted by David Simpson

Guys, Have consulted an ex BT Telecomms Station Manager. Refering to the days before satelite & microwave parabolic antennas, all HF & VHF stations had 3 "Earths" :- 1) Electricity Supplier's Earth - for all building's ring mains, lighting,space heating,etc - and complying with IEE & PME or CEW Regulations earthing & bonding wise. The mains supplied were usually 3ph 4wire star 415V .
2) The Station's "Dirty" Earth :- All rotating machinery, pumps, compressors, s/by gennies etc,installed in an annex or separate building, had it's own dedicated earth.
3) Each Aerial had it's own Earth Matt(Woven Copper Mesh),buried adjacent or under the Aerial pole or mast. All these Matts could be bonded together underground - depending on local earth conductivity conditions.
NONE of these 3 "Earths" were connected to another! This configuration proved for many years to be the best & safest for transmisson & reception purposes

All the earths are bonded together on our radio stations these days; 'earth' is earth. Vast swathes of earth bus-bar connect anything and everything metallic; transmitters, antenna ground-mats and radials, roofs, the mains... The stations are surrounded by earthing rods, all connected to the station bus-bar earth to which all electrical equipment is connected. Even then, a potential difference at RF can appear between them!

Our VLF stations have manhole lids bonded to manhole surrounds, bonded fences, gates and other extraneous metalwork.

I remember at Droitwich in the 1980's there being some 240V 60W lamps between two adjacent transmitter earths; they glowed brilliantly sometimes! I can touch a key to the steel e-LORAN cabin and the earth busbar to which it is bonded a couple of metres away, and demodulate ('rusty bolt effect'), producing the distinctive 'tikkatikkatikkatik...'