Ground cable sizing and resistence

lazza

HI Forum

I have a couple of questions on grounding.

1. What cable size should the ground cable be?
2. What should the resistence of the grounding and ground spike be? .... and how to i measure it?

Thanks
Larry

BB.

Re: Ground cable sizing and resistence

Here is a very nice Fluke reference document on "Grounding" (PDF Download).

Apparently, the proper way to do the testing is to by a USD$1,500+ ground tester (Fluke makes several ). I believe many (most) electrical contractor here in the states should have access to such test equipment.

In the US, the minimum ground cable is 6 awg minimum. However, remember that the cable also has to be sized for the maximum current expected.

DC Battery banks can supply hundreds to thousands of amps--So while a 6 awg ground rod connection may make sense because you (typically) cannot sink 1,000's of amps through a ground rod--You may need a heavier DC System Ground Wire/Bus that can take the maximum current from your larges fuse/breaker (which could be 300-350 Amps for a large DC system).

For example, the fusing current of a 6 awg wire is ~668 amps (from this table). I don't have an NEC table handy, but 6 awg maybe "good enough" for upwards of a 200 amp service. That would lead me to guess that for a 350 amp DC "service", your minimum grounding should be on the order of 3-4x 350 amps or upwards of 1,400 amps (table does not go larger than 6 awg--so an estimate would be ~2x more current or ~3 awg thicker wire--3 or 2 AWG).

I guess 6awg~13.3mm² and 2awg~33.6mm² for those metric impaired folks.

Anyway--Our NEC codes would list this sort of information... I assume that Spain/Europe has similar requirements.

-Bill

Windsun

Re: Ground cable sizing and resistence

What exactly are you grounding, and why?

lazza

Re: Ground cable sizing and resistence

HI

Ok that sounds good- I am planning to use a 16mm2 grounding system... the maximum DC current fuse is only 130A but I want to make sure the ground is optimal.

I am not actually grounding anything. The client wants to put in his own ground rod, and I'm not 100% confident he knows what he is doing, so wanted a way to test whether the resistence was ok. It's on off-grid system at 24V, 2kW inverter.

One more question:

The ground bus (metal plate joining the various grounds ie chassis, inverter neutral & PE etc) does it have to copper, or would a steel or aluminium plate be ok?

Thanks
Larry

BB.

Re: Ground cable sizing and resistence

lazza wrote: »

I am not actually grounding anything. The client wants to put in his own ground rod, and I'm not 100% confident he knows what he is doing, so wanted a way to test whether the resistence was ok. It's on off-grid system at 24V, 2kW inverter.

The first step of good grounding it to ensure that all metal in the home (metal water pipes and even gas pipes/appliances/electrical boxes/etc. Are grounded to the "common" safety ground for the home.

You do not want, for example, a "hot lead" from your DC power or AC inverter output to "energize" a stove (example) and somebody to get shocked moving a pot from the stove to the sink while turning on the kitchen faucet.

So, the safety ground wires need to be cable of "shorting out" any energized metal "enough" to blow a fuse/trip a breaker (i.e., if you had 13 amps from AC inverter branch circuit that could short to the stove (fan hood, electric mixer, stove light, etc.), then the wire needs to be able to draw >13 amps and pop the safety fuse/breaker.

Notice, up to this point I have not discussed the issue of earth ground. And if the home uses plastic pipe and/or has no buried (exposed metal) utilities--Earth ground does not even play a part (so far).

The reason for earth grouding/bonding is (my humble opinion) is to dissapate lightning, static electricity (say a tall antenna/tower that can charge to hundreds or even thoushands of volts--if insulated from earth ground), and to reduce the chance of gavantic corrosion (example, water pipe is made "hot" by an eslectrical connection between battery+ and a grounded earth ground -... This wiould cause the +pipe to corroded and fairly quickly fail).

So, tying all metal together helps ensure the "metal" is all at the same potential. No shocks to people, no corrosion driven by DC (or AC) voltages for inground metal water and gas piping.

Lastly, is the issue with lightning. Towers and Wind Turbines (and even solar arrays) are very good lightning "attractors". So, having a way of directing the energy away from the home (and using good quality surge suppressors on power/telephone/etc. cabling enterning the home/business) to earth is very important. If you have dry/rocky soil--Getting a good ground can be very difficult. Here is some information on lightning grounding... If you have any "HAM" (amateur radio operators/clubs) in the area, they can probably help too.

BB. wrote: »

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's consolidated FAQ page:

www.windsun.com
Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

One more question:

The ground bus (metal plate joining the various grounds ie chassis, inverter neutral & PE etc) does it have to copper, or would a steel or aluminum plate be ok?

Steel is, roughly, about 3x the resistance of copper--So, a steel plate would need to be ~3x thicker than copper.

Aluminum is a better conductor than steel, but aluminum has a problem with corrosion. Basically all aluminum alloys want to form a (very good and hard) insulative "skin" within seconds or minutes upon exposure to air.

Add the fact aluminum also flows under pressure (joint gets hot, flows from electrical joint, cools which allows air back in, forming more corrosion and smaller "good connection", gets hot--repeats.

So aluminum is not really recommended for making "good" electrical connections and the times aluminum wiring has been tried in the US, has resulted in failures of connection and even fires.

In general, only special hardware that "cuts" through the corrosion and forms a "gas tight" connection (special washers, using a "grease" for ground bonding, and special compression connectors for wiring connections).

The best would be to purchase copper or brass plate for ground bus connections. Less trouble in the future.

And, remember to avoid "ground loops". In the US, we ground the AC neutral in the main electrical panel. If you grounded the inverter output at the "Ground Bus" too--that would create alternate paths for ground/neutral return currents (another example would be a connection at the inverter for DC and AC grounding and doing the same thing at the ground bus and/or ground rod).

Hope this all makes sense.

-Bill

Windsun

Re: Ground cable sizing and resistence

I am still not sure if you are talking about an electrical (AC or DC?), ground, the panel mount/frame ground, or?