Camera feeding two DVRs

[seasickstevie]

How can I feed two DVRs from one camera? I know I could feed one, then take the output from that channel to feed the second, but that would add considerably to the cable run. Plus I'll probably want the channel output from each DVR for a monitor.

The camera is on a shared garage between two houses. Both family members. Don't want to put two cameras in the same position if I can help it.

 

[alumni]

Use a BNC 'goalpost' at the camera output and run a cable to each DVR

 

[seasickstevie]

Thanks for the reply. I'd thought that simply using an 'adapter' and presumably taking two lines in parallel would cause a drop in signal. Maybe that's not an issue?

 

[OwainDIYer]

Warning - you may be linking the earthing systems of two separate houses doing this.

 

[expertwitness]

Use the splitter, power the cam from building 1.
Cable to building 2 as stated.
Never had an issue, many installations with split DVR/NVR and shared cameras. At worst use a \"Hum Block\" if the picture has a line through it.
If it were data being sent the I would worry about the \"potential\" of grounding both ends.

 

[seasickstevie]

Warning - you may be linking the earthing systems of two separate houses doing this.

Good point. Thanks for mentioning this.
Although inevitably I would expect everything concerned is going to be class ll.

 

[SimonH2]

Although inevitably I would expect everything concerned is going to be class ll.

But you'll still have a metallic path via the coax from one DVR to the other. Even if both are double insulated, they will be connected to other stuff (eg TV) and so the net of connected stuff spreads until you're likely to find something that's earthed. If there's paths to earth at both ends then you could get currents through a lot of equipment, and either way, there's the safety issue if one or other side has a fault and it takes equipment at the other side to a dangerous voltage, or causes currents of damage causing/fire starting magnitude.

EDIT: A wireless video sender to one side would cure the earth problem, though it does introduce it's own issues.

 

[Wiredup]

How do you explain \"bus\" lines between buildings for access/alarms/computers working and not causing these problems?

 

[OwainDIYer]

How do you explain \"bus\" lines between buildings for access/alarms/computers working and not causing these problems?

They should be using equipment that is property specified and installed.

Just linking systems between buildings runs the risk of importing voltage from a 'foreign' earthing system.

 

[Wiredup]

It is, hence no issues.

 

[SimonH2]

How do you explain \"bus\" lines between buildings for access/alarms/computers working and not causing these problems?

Ever seen the effect of an earth potential difference on non-isolated equipment. Had that at my last place and pretty well every terminal/printer that wasn't in the office with the main system (plus a few that were) was toast - together with the main system.

Under normal conditions, these connections will work, and usually work just fine. But just lose the neutral in one building (for example, and it does happen - although rarely) and (for TN-C-S system) you may find "earth" in one building is at anything between 0 and 240V AC relative to "earth" next door.

Standard advice when taking computer connections (my area) between buildings is always to use isolation. Twisted pair ethernet has transformer isolation (though this has significant capacitive coupling), optical fibre is preferred. RS232 serial is not isolated, and as noted above tends to cause "interesting" mass failures. RS422/423 serial may or may not be isolated. Isolators for most types of connectivity are available.

I've also direct experience of dealing with the problems caused by surges imported via the phone lines. These tend not to damage a "standard" phone which is effectively not connected to anything locally - hence not affected by common mode surges. Add a fax, cordless phone, modem, whatever, that has a local power supply & earth and/or a connection to (eg) a computer which is earthed, and you suddenly find the common mode surge has somewhere to go with frequently destructive results (was a very expensive day for the customer with the fried laptop & printer). The phone and table lamp were unaffected.

EDIT:
I realise all the above has been written from the perspective of the equipment. Now suppose you import 240V AC from next door via your camera cable, and none of your AV kit is earthed (much isn't these days). You now have a TV, possibly satellite receiver, DVD player, and so on - all with their chassis and exposed connectors at 240V AC. The camera may well stop working - at which point you go groping round the back of your equipment to check the plug is in ...
If something in your setup is earthed, then you've now got 240V AC coming in via the camera cable, then via one or more interconnecting AV cables, and eventually to your earth.

 

[Wiredup]

You will note I have also mentioned earth chokes, although called different in the above, have used optical isolators when applicable. I am aware of the \"potential\" differences. Also of different earthing techniques and line rotations. Also well aware of TN-C-S systems.

All here are saying NO. We are not talking a multi camera system, say 1000 cams on 60 odd buildings. We are talking about two houses probably built at the same time.
Even interlinked Fire systems between buildings, using class change?
I repeat this is one camera between two buildings probably built at the same time. You can add fuses/chokes/isolators if you want but it will not be needed in this instance.

 

[bernardgreen]

error in edit deleted

 

[bernardgreen]

I repeat this is one camera between two buildings probably built at the same time. You can add fuses/chokes/isolators if you want but it will not be needed in this instance.

When the houses were built is irrelevant. The relevalent matter is whether they are are supplied with "earths" derived from the same network neutral conductor in the stree or from two neutrals in different branches of the network.

I have lost count of the number of opto-coupled isolators I have installed on data links, even between factory floor and production office in the same building after damage to equipment caused by high currents in the signal ground conductor. Damage that was considered to be impossible with equipment installed.

For this application to ensure equipment is not going to suffer damage at some time in the future isolation is necessary

Very simple home made solution for video

http://www.extremecircuits.net/2010/05/video-isolator-circuit-diagram.html

Note the comment

since the voltages across these capacitors can be higher than you might expect (A PC that isn’t connected to the mains Earth can have a voltage as high as 115 V (but at a very low, safe current), caused by the filter capacitors in its power supply.

That voltage difference is a hazard when exported to a remote location.

 

[SimonH2]

Just a thought ...

There's a slight polarisation between "just do it, it'll work" and "you need to do <something> to avoid danger".

Well both are right. You can almost certainly just connect them together, and almost certainly nothing untoward will happen. but, a large proportion of (in this instance) the Wiring Regs are dedicated not to what happens normally, but what happens when something abnormal happens.
For example, there are tables/calculations regarding the size of fuses/circuit breakers to supply a given size of cable - not because the fuse/circuit breaker is needed to make whatever is connected work, but to deal with abnormal situations (ie disconnect the supply under fault conditions rather than set fire to the cable). The system would work without a single fuse/breaker between the main supply and the sockets or lights - but if a fault happens then you'd have a big problem.

So, yes, connecting this camera directly to two recorders in two houses will almost certainly work without any problems. It will almost certainly work without problems for years. There most likely will never be a problem like the ones being talked about. But do you design a system on the basis of ignoring risks that most likely won't happen ? Or do you take them into account ?

While thinking about that ...
Most people don't have an accident in their car - the law only requires third party insurance, yet the majority have comprehensive insurance.
House fires are fairly uncommon (most people will never have a house fire during their lives), explosions less common still. Most people have insurance for their house.