Project Vivat – Power System considerations

By Richard Harris

Introduction

There is more to consider when providing power for an outside broadcast unit than arriving on-site and plugging into the nearest socket outlet. Safety and compliance with regulations concerning use of power need to be taken into account, particularly when the public are present. There are some complex technical and legal issues involved.

An OB unit will draw power from outside an existing building or from a mobile generator. This establishes a separate utilisation area, generally outside the equipotential earth area of the source and the situation is complicated when power is taken back into the primary area (building) for operating auxiliary equipment e.g. monitors.

For an organisation such as BECG, the activity does not create a “Place of Work” and the usage may be considered a controlled technical area, allowing concessions for certain items, such as use of EP-4 connectors. Having highly qualified engineer members does not guarantee safety, and compliance, particularly as not all engineers are fully conversant with all disciplines, practices and regulations.

History

In the 1950s (MCR 13 to 16 by Marconi) and 60s (MCR 19 to 28 by Pye), BBC Outside Broadcast Units had a similar power supply design policy, which was carried on through into the Type 2 and Type 5 CMCRs.

A portable Intake Switch Unit (ISU) was located at the source of supply, fitted with a double pole switch and BS88 fuse.

The mains supply intake to the OB unit generally had an 80 Amp BICC power input connector. These were two pole, with an earth connection via the steel casing. A 100Ft (30 Metre) cable, with moulded-on connectors, cost over £1,000 in 1987 and weighed about 155 lbs when reeled on a drum – but they are now obsolete.

There may have been a second, standby, 80A input or a “loop-out“ socket for an associated vehicle such as radio links. Several 15 Amp “Niphan” metal body sockets were fitted, for power back into the location, for monitors, crane pumps etc.

ITV companies and independent facility providers used similar systems.

The BBC return feeds used 3 core cable with a non-current-carrying metallic outer screen (Live, Neutral, Earth, Screen/Chassis) thus providing a low impedance equipotential earth contained environment.

RCCDs (residual current circuit devices) were not in common use at that time and were considered an impediment for broadcast use, due to the risk of nuisance tripping. From the point of view of compliance with electrical safety regulations an OB unit was considered to be a kind of caravan, being subject to frequent connection and disconnection. Local earth spikes were generally used and fault current control was by local main circuit breakers and BS-88 fuses at the source of supply.

Original-MCR-panel-exterior
Original MCR intake panel exterior.
Photo: Richard Harris.
Original-MCR-panel-interior
Original MCR intake panel interior.
Photo: Richard Harris.

Planning

It was necessary to consider if reconstruction of an outside Broadcast vehicle could be carried out using the original power philosophy. Given that it was intended to connect the vehicle to modern systems at venues and sites, themselves compliant with current wiring and safety standards, it was concluded that a more up-to-date approach was needed. The BICC connectors are no longer made and were only ever available as a factory fit. The Niphan sockets are quite unsuitable and had several operational shortcomings.

Project Vivat is to re-create a 1950s Outside Broadcast Unit, similar to MCRs 13 to 16, which were in use at the time of The Queen’s Coronation and Churchill’s Funeral. None of the original vans have survived but a very similar 1960s vehicle, for which there are effectively no cameras remaining, was available as a base.

The original power intake panel, internal distribution and all wiring were all missing, so we had to build the power system from scratch. A modern intake and primary power control panel was built to feed period internal distribution panels which were re-furbished and modified to better meet current standards.

The new installation

The intake panel has a main input on 63A CEE17 (Blue) plug and a night supply on a 16A CEE17 plug. The night supply circuits can be switched to be fed from the main supply. Local main intake isolator switches are provided, and metering and incorrect polarity indicators are fitted.

The supplies can be protected by external RCCD at the point of connection if necessary. An earth terminal is provided, bonded to both chassis and incoming earth. Loop out and standby inputs were not considered necessary as these could be dealt with by external distribution and avoided several safety and operational issues.

All outgoing circuits to the internal distribution are protected by MCB (minature circuit breaker). Three 16A CEE17 sockets are provided for “on-site” equipment. Each is protected by MCB and RCCD. Any equipment in use away from, and supplied from, the OB is therefore RCCD protected.

Power-Input-Panel
Vivat intake panel exterior.
Photo: Dave Hill.
Back-of-the-panel
Vivat intake panel interior.
Photo: Dave Hill.

It may be worth pointing out that the use of RCCDs satisfies many details of the regulations, but they should not be considered an absolute safety feature. A trip cannot distinguish between a piece of equipment and a person connected between phase and neutral, it only operates when there is sufficient leakage to ground. Further, the sensitivity (normally 30mA) can be compromised by the effective power factor of the equipment connected. This is particularly significant when phase-controlled lighting dimmers are in use. Effective earthing, resulting in a low Earth Loop impedance, is essential allowing adequate fault current to flow for the protective devices to operate. (A loop impedance of 0.6 ohms was required for a 63A BS88 fuse protected circuit).

The vehicle has a voltage regulator (AVR) to feed the technical equipment, which can be bypassed if necessary. A control panel inside the vehicle carries metering and two circuit breakers for the controlled supplies, which are divided into four areas: Engineering, Monitor stack, Production desk and auxiliary equipment rack. The breakers are readily available to the operators, so that the equipment can be shut down quickly in the event of a serious problem – remember, some of the equipment is over 70 years old. The internal technical distribution uses Canon EP-4 connectors. A second (Marconi) panel controls “domestic” circuits such as lighting, battery charger, extraction fans and utility sockets, via circuit breakers and small fuses.

Vivat-Interior-Panels
Interior Panels, AVR control and distribution above, Auxiliary services panel below (Dugdills work lamp on the left hand side, for maintenance space below).
Photo: Dave Hill

Canon EP-4 Connectors

Much of the internal equipment dates from the 1950s and is fitted with Canon EP-4 mains input plugs. These were phased out of use in the mid 80s for various reasons including un-shuttered contacts and low creepage/clearance distances. However, it has been decided not to change these as it would impair the historical aspects of the equipment. All are checked and additional earth bonding added. The situation is further complicated as BBC / EMI used a different wiring standard to Marconi, but as most of the equipment is Marconi this standard has been adopted. There are some modern XLR, IEC and Powercon mains connectors in use, for convenience, where they are not seen such as behind the auxiliary equipment rack.

3 Phase

Some large modern vehicles run from 3 phase supplies which enables equipment like air conditioning units to operate more efficiently. However, in the 1960s a single phase supply of sufficient capacity could generally be made available and the units, despite the extra load of valve based equipment, were smaller, so a single phase supply was preferred for simplicity, safety and less problems with synchronising – remember that the video signal was transmitted on an unbalanced co-axial cable and synchronised to 50Hz mains.

The Project Vivat vehicle intake is single phase only and, with an estimated maximum load of 50A, a 63A capacity supply should be sufficient to power the vehicle and its immediate associated equipment such as commentators’ monitors. If additional power was required for an OB, a distribution from the source would need to be employed. (e.g. 125A single phase or 63A 3 phase).

Cable and Wiring

The vehicle has built-in twin-tray cable ducts, but these cannot really be treated as electrical trunking due to inevitable shared use and the possibility that single cable wired circuits could become separated. Mains circuits will be wired with double insulated flexible cables, generally PVC, with integral CPC (circuit protective conductor), but larger cross-sectional area cables are HO-7 spec. rubber. The intake panel was built as a sub assembly with triple-rated switchgear cable in finger trunking. Some wiring, for example feeds to overhead lighting, make use of existing conduit where practical.

Battery System

The original vehicles had several battery systems, 24 volts for relays and signalling, 24 volts for inverter fed internal lighting, 42 volts for sound and 12 volts for the vehicle road systems. Programmes could continue with sound only in the event of a mains supply failure.

The production area lighting is fluorescent tubes. A modern inverter/charger unit and a 12 volt battery are planned, located in a skirt locker, to give internal lighting when mains is not available. Although consideration will be given to conversion of the fittings to LED in the future, on the basis of reduction of power consumption, possible reduced availability of replacement tubes and improved performance, this must offset against loss of originality.

There are no external rigging lights, but many of the lockers have timer-controlled lighting. The cab and its overhead lockers have battery lighting and a 12 volt socket has been added.

A charger, for the vehicle battery, is located in a rear locker alongside the AVR. There are no plans for separate signalling supply or sound equipment supply, partly to reduce the maintenance overhead of the extra batteries.

Acknowledgements

Paul Hundy donated the Marconi internal mains distribution panel, which came from Alpha Television.
The AVR and its control panel came from ABC/Thames Television.
The Hayberd battery charger was bought, as an unused item.
Brian Summers of BTTT provided the intake panel mounting frame. 
Sam Booth donated the Dugdills work lamp for the internal control area.

Original (MCR-21) Panels: Photos by Richard.
Vivat Panels: Photos by Dave.