12v Electrics - Negative, Return, Ground & Screen

[Blue Drifter]

Hi all - grateful for some advice on subject issue. I suspect that negative and return are one and the same but can someone give a brief explanation of ground and screen as applied to connecting instruments such as chart plotters etc. Many thanks in advance

 

[Tintin]

Hi all - grateful for some advice on subject issue. I suspect that negative and return are one and the same but can someone give a brief explanation of ground and screen as applied to connecting instruments such as chart plotters etc. Many thanks in advance

Connect them all together. It should be fine.

 

[cimo]

hello DB, if nothing else, this will bump your post.

I always consider negative to be a term exclusive to DC systems.
Nowadays, Return & Ground appear to be used commonly across DC and AC systems. Although Return & Ground are usually the same thing for DC, they have distinct functions for AC (return=neutral & ground=Earth).
Earth, we tend to use exclusively with reference to AC systems (although people do use it in reference to DC also!).

As regards connecting instruments in a pleasure boat, ground is the ships ground, which, more often than not, is also the battery negative. The engine block is likely to be at ground potential (i.e. connected to the battery -ve terminal), so you could say that the engine block is the ships ground (which is also the return). If this is making sense so far, you'll be delighted to learn that some systems have an isolated return (isolated from the ground that is), but I ain't going into that now. Interesting to note also, some older machines (tractors and such like), employed a positive ground, where the battery positive terminal (rather than the negative) was connected to the engine block (but that’s another story also).

Your ground will likely be the negative busbar (or common connection point) close to you switch panel. You will likely find that this busbarr is connected to the engine block also (do check for yourself).
As regards the screen connection, this connects to the negative busbar also.
The purpose of the screen is to ground any radiated electromagnetic fields which may cross interfere with neighbouring equipment and/or cabling. The screen is typically grounded at one end only, (to avoid ground loops, which can in themselves act like antenna, receiving radiated interference!).

It's clear as mud.

There will be others along with different ways of saying the same thing I'm sure.

 

[Blue Drifter]

Thank you for your responses

Thanks for the replies - it does make a difference and adds substantially to my knowledge and confidence in dealing with these issues.

 

[nigelmercier]

Negative: the negative busbar, usually the battery negative or a shunt.
Return: same thing but in the USA.
Ground: normally the safety conductor in an AC system, sometimes used for DC systems or amplifiers etc.
Screen: the braided conductor that protects against interference in low-level signals, or in signals that are otherwise susceptible to interference. This is often connected to negative, but not always.

 

[William_H]

hello DB, if nothing else, this will bump your post.

I . The screen is typically grounded at one end only, (to avoid ground loops, which can in themselves act like antenna, receiving radiated interference!).

It's clear as mud.

There will be others along with different ways of saying the same thing I'm sure.

Ground loops. The reason that often it is desirable to earth only one end of the screen is that the screen if earthed both ends can end up carrying some of the supply current. This current especially in AC or DC in amplifier circuits can be varying a lot with noise. If this noise is carried through the screen it can end up inducing noise into the signal wire. So earth only one end so no power current in the screen. Just to explain. olewill

 

[Tintin]

Ground loops. The reason that often it is desirable to earth only one end of the screen is that the screen if earthed both ends can end up carrying some of the supply current. This current especially in AC or DC in amplifier circuits can be varying a lot with noise. If this noise is carried through the screen it can end up inducing noise into the signal wire. So earth only one end so no power current in the screen. Just to explain. olewill

+1

 

[cimo]

Ground loops. The reason that often it is desirable to earth only one end of the screen is that the screen if earthed both ends can end up carrying some of the supply current. This current especially in AC or DC in amplifier circuits can be varying a lot with noise. If this noise is carried through the screen it can end up inducing noise into the signal wire. So earth only one end so no power current in the screen. Just to explain. olewill

thanks for the correction William. focusing on my spelling lately, my grammar appears to have suffered!

to rephrase -
The screen is typically grounded at one end only, (to avoid ground loops. Grounding one end can make the screen itself act like antenna, receiving radiated interference!).

 

[Docktor]

What is the difference between Earth Return and Above Earth?

Also, what is the difference between Common Ground and Insulated Return?

How do you tell which type of system you boat uses?

 

[Docktor]

Bump! Does anyone know what these ground states are specifically? It's very confusing and I'd appreciate any responses! Thanks!

 

[cliff]

What is the difference between Earth Return and Above Earth?

Also, what is the difference between Common Ground and Insulated Return?

How do you tell which type of system you boat uses?

Common ground - think motor car - chassis / body - common ground / return for multiple items with the return connected to the body / chassis adjacent to teh fitting or even through the fitting itself.
Insulated return - think twin core cable with the return - (-ve) taken back to the battery / power source. Some alternators and starter motors are insulated return - ie they are not electrically connected to the engine block hence the block has no electrical potential imposed on it - very important for Aluminium hulled boats and to a lesser extent steel hulls.

GRP/Wooden boats do not suffer the same corrosion problems and can cope happily with using the engine block as a common ground for the starter motor and alternator - just make sure your shaft anodes are in good nick.

It is normal on a boat to have the "domestic" 12/24v fittings (lights etc) wired as insulated return back to a common bus bar then one heavy return cable back to the battery bank (or in some cases back to the and then to the battery bank if the starter motor and alternator are chassis / frame return)

 

[Docktor]

Thanks Cliff - that's clear as crystal! Appreciate the reply and the use of examples that really help!