3 phase power

[Deleted member 121002]

I work in Broadcast television as a broadcast systems designer. I have seen in recent months a trend that is worrying (not sure if it is legal) that in a 19" rack there could be several phases wired to power distribution outlets (normally via IEC connectors) This means it is imperative that colour coding and wiring practices are set in concrete if two pieces of equipment within inches of each other are powered from different phases. I always took the view that the phases should be separated by at least 2 metres or as it was recently that the racks in a row would have been wired Red, then the next rack would be Yellow and then Blue, to avoid accidental crossing of phases. I welcome all information and guidance on this subject and if you can point me to the right place for current documentation, my IEE wiring regs does not show this current trend.

 

[Bobster]

Nothing wrong with it.

How could you accidentally cross a phase with an IEC connector?

 

[Lucien Nunes]

This is mainly a historic requirement. Regulation A20 of the 14th edition required all single-phase socket outlets in any one room to be on the same phase, except in commercial / industrial locations where this was not practical, in which case a separation of 6 feet was required between single-phase sockets on different phases. It was often rigidly enforced in the theatre, with each bar or area on one phase and a prohibition on running extensions from one area to another. With the advent of better insulation especially on connectors, e.g. insulated plug pins, and general reduction in shock risk, the requirement was waived. If each phase is protected by a single-phase RCD, line-line faults and possible shock paths are monitored by multiple devices simultaneously, further reducing the risk.

For analogue audio / video there was a functional advantage of minimising the possibility of interference at the 3rd harmonic, which is much more likely to manifest as audible or visible hum on signal. We still tend to avoid splitting production audio loads across phases for this reason, although for exclusively digital signals there is no advantage in doing so.

 

[Pete999]

I work in Broadcast television as a broadcast systems designer. I have seen in recent months a trend that is worrying (not sure if it is legal) that in a 19" rack there could be several phases wired to power distribution outlets (normally via IEC connectors) This means it is imperative that colour coding and wiring practices are set in concrete if two pieces of equipment within inches of each other are powered from different phases. I always took the view that the phases should be separated by at least 2 metres or as it was recently that the racks in a row would have been wired Red, then the next rack would be Yellow and then Blue, to avoid accidental crossing of phases. I welcome all information and guidance on this subject and if you can point me to the right place for current documentation, my IEE wiring regs does not show this current trend.

Three phase colours have changed, no longer RYB and Black for Neutral if you intend to derive Single phase supplies. see Table 51 BS 7671 17th Edition

 

[Strima]

Very common to use three phase in telecoms racks but it can be a bit of a pain to balance phases with people plugging in random items with no thought:

 

[Deleted member 121002]

Many thanks for the replies, better than I dared hoped for.
The 3 phase PDU's installed (power distribution units) have 3 banks of IEC connectors and are labelled Circuit 1, 2 and 3. Not very clear which phase is which though. Each piece of equipment often has (not always) 2x power supplies, main and back up. My concern is that with an A PDU (3 phase) and B PDU (3 phase) one actually has to be 100% certain that if the equipment is connected to A PDU circuit 1 IEC skt 1, then the back up PSU MUST be connected to the B PDU circuit 1 skt 1.
The wiring is done this way BUT there is no warning labels in the racks to show this set up. This is my 3rd day here and I am going to change/add more labels especially for the 3 phase in the racks. I need to discuss this with upper tech managers to make sure they understand the problem. Many thanks for everyones help and guidance. I will take some pictures of the PDU and associated wiring for you to see the set up.

 

[Deleted member 121002]

I will also get myself a copy of the latest wiring regs

 

[Strima]

A lot of dual few equipment tends to feed into switch mode power supplies which are self contained, this removes the need for the equipment to be few from the same phase and frequency.

 

[Deleted member 121002]

Nothing wrong with it.

How could you accidentally cross a phase with an IEC connector?

Hello Rob, you're right in what you say. But since the equipment has 2x power supplies fed via IEC connectors. Both PSU's must be fed from different UPS's but on the same phase. In our case it could be very easy to get this wrong and have 2 different phases in the same piece of equipment, hence warning labels in the back of the racks.
Thank you for your reply.

 

[Spoon]

There should be a copy of the wiring diagrams in these racks.

I usually do a single line diagram that is put on the inside door of cabinets, for easy reference.

 

[davesparks]

How would feeding the backup PSU from a different phase be a problem?
Surely it is favourable to feed the backup from a different phase in case of one phase being lost?

 

[Deleted member 121002]

There should be a copy of the wiring diagrams in these racks.

I usually do a single line diagram that is put on the inside door of cabinets, for easy reference.

Normally in Broadcast kit, wiring diagrams are kept away from the racks, but often copies are pinned to walls for easy reference. BUT in our case, where we have 3 phase feeds into one rack (A + B) I will add more warning labels and colour code them as well.

 

[Deleted member 121002]

A lot of dual few equipment tends to feed into switch mode power supplies which are self contained, this removes the need for the equipment to be few from the same phase and frequency.

Hi, Most PSU's are indeed switch modes and since they are "floating" until the DC has been converted from the hi frequency supply. That said, it is common practice to use the same phase for A and B UPS feeds. In the case of an dual mains input MDU (mains distribution unit) most of these units have zero crossing switching (current frequency) to minimise any hiccups when switching between A or B feeds, also it means there is minimal current be used when switching. If these were different phases this would not be possible. Plus there is the inherent voltage to contend with between phases of 400v. It is safer to make sure the system is installed so everyone is on the same phase (in a desk or rack) than accept that the kit could work ok between different phases. We just cannot take that risk.

 

[James]

If you are using a double conversion UPS i.e. ac to dc to ac
The voltage between any outgoing cable of 1 UPS could be 400v different from an outgoing cable of a second UPS
As a general rule, neither UPS will sync it’s outgoing frequency to either incoming mains or a secondary UPS
Therefore, frequency difference (very small) will likely shift the phase angle between the 2 UPS units outputs.

There are some special circumstances where the above will not apply.

 

[Deleted member 121002]

How would feeding the backup PSU from a different phase be a problem?
Surely it is favourable to feed the backup from a different phase in case of one phase being lost?

The electrical system in all studios and play out centres are as follows;
The incoming three phase feeds 2x (or more) UPS systems via fuse and switch boards. Each UPS can (or should) power the entire station in the event of a power failure. Most UPS systems will only have enough power to run for an hour or so but long enough for a diesel generator to power up and take over the load but still via the UPS systems. The UPS is always in circuit.